After some fiddling around with debug flags (Ctrl+Shift+F) in BOINC Manager (boinc-client-7.16.6-3.fc31.x86_64) I deactivated the debug flags again and later on I closed the BOINC Manager to pay a visit to the dentist. (They measured my temps, I needed to wash my hands twice and rinse my mouth for 30 seconds with some desinfectant H₂O₂ solution, and I had to sign a document that I didn't have any signs of the coronavirus.) When I came back, I started the BOINC Manager again, but something was completely going wrong there, it was playing havoc with the tasks in the queue: instead of displaying the Tasks queue, its window was constantly alternating between displaying one New task from 01 Jan 1970 00:00:00 and showing the normal queue each second. I tried to solve that problem by trying to set some debug flag (e.g. http_xfer_debug) but even after saving the new setting, it didn't catch the new setting. It just wouldn't stick. So, after a quick deliberation I decided to move the file ~/.BOINC Manager out of the way and start from scratch. Well, that also didn't work, because this new instance would start in the Simple View mode and then crash altogether after about one second. I tried that again and again until I managed to type Ctrl+Shift+A for the Advanced View and then it would stick around, at last. Good. Then you need to shift the widths of the columns, you know the drill.

However, to my dismay I also noticed that the Deadline date column was not my favourite one, displaying 'PM' etc. (I'm still having a hard time to understand that notation system, AM and PM, it's costing me way too much time (ha!) to switch back and forth between/to the normal 24-hour clock. It boggles the mind.) That said, I prefer the 24-hour clock. Pure, plain and simple. (...) Anyway, I needed to compare the old ~/.BOINC Manager file with the new one and noticed this one difference:
LanguageISO=en_GB
and
LanguageISO=en_US

This is it! You just need to change en_US into en_GB and it's done. Moreover, I never understood why I couldn't change the language setting in BOINC Manager on my laptop that I took with me on my trip to New Zealand, a few years ago. And now I finally succeeded by changing the value of LanguageISO in that file, ~/.BOINC Manager. Haha!