If anyone got a clue what happens or what to improve in the App, I'd be glad to hear.
BM
One could always improve "Windows", or more specifically Windows system restore behaviour. IIRC default behaviour is to watch and eventually restore the whole folder "program files". You could move (or fresh install) Boinc somewhere else, that is: not in the "Program Files" folder (and of course neither in the "windows" folder). Or you can change the Windows settings for "system restore" to not touch the "boinc" folder; or (maybe not what you'd want to do) not include the whole "program files" folder in the "system restore" settings.
RE: If anyone got a clue
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One could always improve "Windows", or more specifically Windows system restore behaviour. IIRC default behaviour is to watch and eventually restore the whole folder "program files". You could move (or fresh install) Boinc somewhere else, that is: not in the "Program Files" folder (and of course neither in the "windows" folder). Or you can change the Windows settings for "system restore" to not touch the "boinc" folder; or (maybe not what you'd want to do) not include the whole "program files" folder in the "system restore" settings.