By now, I hope that you have managed to fix the problem yourself. I encountered the same problem on one of my machines yesterday and, like you, tried uninstalling and reinstalling etc.

One thing you didn't say is how you were running BoincManager.

Until recently, I had always done a 'standard' install but decided to try installing BOINC as a service on a new build instead. I have now done this on two machines and things have been fine for about a week.

It is on one of those machines that I encountered the same problem as you and after rereading your comment about BOINC never completely uninstalling (which is good in that your current WUs are never removed), I had an inspiration to force BOINC to behave diffrently because simply uninstalling then reinstalling didn't appear to do the trick.

For me, I now have BOINC running again (and connecting to localserver) by reinstalling it with the standard settings. This has now been running for about 12 hours so I think its safe to say that the problem is resolved for me.

If you were running it in standard configuration, try re-installing it as a service and see if that fixes the problem. Later you could change back if you prefer (untested by me).

Let us know if that fixes your problem, and Good Luck!