After receiving message "Server can't open database" in answer to a fetch Boinc said it received a "none" profile and stopped trying to fetch work.
After manually updating it, work resumed normally and the correct profile was received.

Message log follows. Unfortunately I have forgotten to save the file "sched_reply_www.worldcommunitygrid.org.xml" before updating.
Few weeks ago after finding Boinc's queue empty without any other message I had found the above file with a 0-byte length...

12-Mar-2008 07:58:59 [---] Starting BOINC client version 5.10.45 for windows_intelx86
12-Mar-2008 07:58:59 [---] log flags: task, file_xfer, sched_ops, checkpoint_debug
12-Mar-2008 07:58:59 [---] Libraries: libcurl/7.18.0 OpenSSL/0.9.8e zlib/1.2.3
12-Mar-2008 07:58:59 [---] Data directory: C:\WCG\BOINC
12-Mar-2008 07:59:00 [---] Processor: 4 GenuineIntel Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz [x86 Family 6 Model 15 Stepping 11]
12-Mar-2008 07:59:00 [---] Processor features: fpu tsc pae nx sse sse2 mmx
12-Mar-2008 07:59:00 [---] OS: Microsoft Windows XP: Home Edition, Service Pack 2, (05.01.2600.00)
12-Mar-2008 07:59:00 [---] Memory: 1.99 GB physical, 3.84 GB virtual
12-Mar-2008 07:59:00 [---] Disk: 15.98 GB total, 4.44 GB free
12-Mar-2008 07:59:00 [---] Local time is UTC +1 hours
12-Mar-2008 07:59:00 [---] Version change (5.10.42 -> 5.10.45)
12-Mar-2008 07:59:00 [WCG] URL: http://www.worldcommunitygrid.org/; Computer ID: 504417; location: work; project prefs: work
12-Mar-2008 07:59:00 [---] General prefs: from World Community Grid (last modified 29-Feb-2008 18:07:58)
12-Mar-2008 07:59:00 [---] Host location: work
12-Mar-2008 07:59:00 [---] General prefs: using separate prefs for work
12-Mar-2008 07:59:00 [---] Reading preferences override file
12-Mar-2008 07:59:00 [---] Preferences limit memory usage when active to 1528.53MB
12-Mar-2008 07:59:00 [---] Preferences limit memory usage when idle to 1528.53MB
12-Mar-2008 07:59:00 [---] Preferences limit disk usage to 3.98GB
12-Mar-2008 07:59:13 [---] Running CPU benchmarks
12-Mar-2008 07:59:44 [---] Benchmark results:
12-Mar-2008 07:59:44 [---] Number of CPUs: 4
12-Mar-2008 07:59:44 [---] 2346 floating point MIPS (Whetstone) per CPU
12-Mar-2008 07:59:44 [---] 4907 integer MIPS (Dhrystone) per CPU
12-Mar-2008 07:59:45 [WCG] Restarting task dddt0401n0834_100133_1 using dddt version 515
12-Mar-2008 07:59:45 [WCG] Restarting task dddt0401n0834_100224_0 using dddt version 515
12-Mar-2008 07:59:46 [WCG] Restarting task dddt0401n0836_100048_0 using dddt version 515
12-Mar-2008 07:59:46 [WCG] Restarting task dddt0401n0837_100062_0 using dddt version 515
---> Many lines of normal activity deleted
12-Mar-2008 13:08:20 [WCG] Sending scheduler request: To fetch work. Requesting 3 seconds of work, reporting 2 completed tasks
12-Mar-2008 13:08:25 [WCG] Scheduler request succeeded: got 0 new tasks
12-Mar-2008 13:08:25 [WCG] Message from server: Server can't open database
12-Mar-2008 13:08:25 [WCG] New host venue:
12-Mar-2008 13:08:25 [---] General prefs: from World Community Grid (last modified 29-Feb-2008 18:07:58)
12-Mar-2008 13:08:25 [---] Host location: none
12-Mar-2008 13:08:25 [---] General prefs: using your defaults
12-Mar-2008 13:08:25 [---] Reading preferences override file
12-Mar-2008 13:08:25 [---] Preferences limit memory usage when active to 1528.53MB
12-Mar-2008 13:08:25 [---] Preferences limit memory usage when idle to 1528.53MB
12-Mar-2008 13:08:25 [---] Preferences limit disk usage to 3.52GB
---> Many [checkpoint_debug] lines deleted
12-Mar-2008 13:59:42 [WCG] Computation for task dddt0401n0837_100010_1 finished
12-Mar-2008 13:59:44 [WCG] Started upload of dddt0401n0837_100010_1_0
12-Mar-2008 13:59:44 [WCG] Started upload of dddt0401n0837_100010_1_1
12-Mar-2008 13:59:49 [WCG] Finished upload of dddt0401n0837_100010_1_0
12-Mar-2008 13:59:58 [WCG] Finished upload of dddt0401n0837_100010_1_1
---> Several [checkpoint_debug] lines deleted
12-Mar-2008 14:03:13 [WCG] Sending scheduler request: Requested by user. Requesting 7427 seconds of work, reporting 3 completed tasks
12-Mar-2008 14:03:18 [WCG] Scheduler request succeeded: got 1 new tasks
12-Mar-2008 14:03:18 [WCG] New host venue: work
12-Mar-2008 14:03:18 [---] General prefs: from World Community Grid (last modified 29-Feb-2008 18:07:58)
12-Mar-2008 14:03:18 [---] Host location: work
12-Mar-2008 14:03:18 [---] General prefs: using separate prefs for work
12-Mar-2008 14:03:18 [---] Reading preferences override file
12-Mar-2008 14:03:18 [---] Preferences limit memory usage when active to 1528.53MB
12-Mar-2008 14:03:18 [---] Preferences limit memory usage when idle to 1528.53MB
12-Mar-2008 14:03:18 [---] Preferences limit disk usage to 3.51GB
12-Mar-2008 14:03:20 [WCG] Started download of dddt0401o0844_100468_dddt0401o0844_100468.gpf
12-Mar-2008 14:03:20 [WCG] Started download of dddt0401o0844_100468_dddt0401o0844_100468.dpf
12-Mar-2008 14:03:22 [WCG] Finished download of dddt0401o0844_100468_dddt0401o0844_100468.gpf
12-Mar-2008 14:03:22 [WCG] Started download of dddt0401o0844_100468_ZINC06760156-0000.pdbqt
12-Mar-2008 14:03:23 [WCG] Finished download of dddt0401o0844_100468_dddt0401o0844_100468.dpf
12-Mar-2008 14:03:23 [WCG] Finished download of dddt0401o0844_100468_ZINC06760156-0000.pdbqt
12-Mar-2008 14:03:23 [WCG] Started download of dddt0401o0844_100468_0401.pdbqt
12-Mar-2008 14:03:23 [WCG] Started download of dddt0401o0844_100468_AD4_parameters.dat
12-Mar-2008 14:03:24 [WCG] Finished download of dddt0401o0844_100468_0401.pdbqt
12-Mar-2008 14:03:24 [WCG] Finished download of dddt0401o0844_100468_AD4_parameters.dat
12-Mar-2008 14:03:24 [WCG] Started download of dddt0401o0844_100468_ZINC06760159-0000.pdbqt
12-Mar-2008 14:03:24 [WCG] Started download of dddt0401o0844_100468_ZINC01604286-0000.pdbqt
12-Mar-2008 14:03:25 [WCG] Finished download of dddt0401o0844_100468_ZINC06760159-0000.pdbqt
12-Mar-2008 14:03:25 [WCG] Finished download of dddt0401o0844_100468_ZINC01604286-0000.pdbqt
12-Mar-2008 14:03:25 [WCG] Started download of dddt0401o0844_100468_ZINC01604285-0000.pdbqt
12-Mar-2008 14:03:25 [WCG] Started download of dddt0401o0844_100468_ZINC01463265-0000.pdbqt
12-Mar-2008 14:03:26 [WCG] Finished download of dddt0401o0844_100468_ZINC01604285-0000.pdbqt
12-Mar-2008 14:03:26 [WCG] Finished download of dddt0401o0844_100468_ZINC01463265-0000.pdbqt
12-Mar-2008 14:03:26 [WCG] Started download of dddt0401o0844_100468_ZINC07650845-0000.pdbqt
12-Mar-2008 14:03:27 [WCG] Finished download of dddt0401o0844_100468_ZINC07650845-0000.pdbqt
12-Mar-2008 14:03:28 [WCG] Starting dddt0401o0844_100468_0
12-Mar-2008 14:03:28 [WCG] Starting task dddt0401o0844_100468_0 using dddt version 515
12-Mar-2008 14:03:42 [WCG] [checkpoint_debug] result dddt0401o0841_100107_0 checkpointed
12-Mar-2008 14:04:08 [WCG] [checkpoint_debug] result dddt0401n0840_100176_0 checkpointed
12-Mar-2008 14:04:19 [WCG] Sending scheduler request: To fetch work. Requesting 3189 seconds of work, reporting 0 completed tasks
12-Mar-2008 14:04:25 [WCG] Scheduler request succeeded: got 1 new tasks
12-Mar-2008 14:04:27 [WCG] Started download of dddt0401o0844_100250_dddt0401o0844_100250.gpf
12-Mar-2008 14:04:27 [WCG] Started download of dddt0401o0844_100250_dddt0401o0844_100250.dpf
12-Mar-2008 14:04:28 [WCG] Finished download of dddt0401o0844_100250_dddt0401o0844_100250.gpf
12-Mar-2008 14:04:28 [WCG] Started download of dddt0401o0844_100250_ZINC05516532-0000.pdbqt
12-Mar-2008 14:04:29 [WCG] Finished download of dddt0401o0844_100250_dddt0401o0844_100250.dpf
12-Mar-2008 14:04:29 [WCG] Finished download of dddt0401o0844_100250_ZINC05516532-0000.pdbqt
12-Mar-2008 14:04:29 [WCG] Started download of dddt0401o0844_100250_0401.pdbqt
12-Mar-2008 14:04:29 [WCG] Started download of dddt0401o0844_100250_AD4_parameters.dat
12-Mar-2008 14:04:30 [WCG] Finished download of dddt0401o0844_100250_AD4_parameters.dat
12-Mar-2008 14:04:30 [WCG] Started download of dddt0401o0844_100250_ZINC05516531-0000.pdbqt
12-Mar-2008 14:04:31 [WCG] Finished download of dddt0401o0844_100250_0401.pdbqt
12-Mar-2008 14:04:31 [WCG] Finished download of dddt0401o0844_100250_ZINC05516531-0000.pdbqt
12-Mar-2008 14:04:31 [WCG] Started download of dddt0401o0844_100250_ZINC05516530-0000.pdbqt
12-Mar-2008 14:04:31 [WCG] Started download of dddt0401o0844_100250_ZINC00706530-0000.pdbqt
12-Mar-2008 14:04:32 [WCG] Finished download of dddt0401o0844_100250_ZINC05516530-0000.pdbqt
12-Mar-2008 14:04:32 [WCG] Finished download of dddt0401o0844_100250_ZINC00706530-0000.pdbqt