This is to add to a previous bug report I filed, because after some even deeper investigation I figured out the problem, and it may be important to other Krita users. I looked back on the newly updated programs and features to find out the new issue with saving files - as it would happen, the newest release of Bitdefender Total Security, which automatically installs for Windows 10 users, blocks Krita from saving it what it deems "protected" folders like documents and pictures. To users that have both Krita and Bitdefender, you now have to allow Krita as an exclusion to Bitdefender file protection, or any attempt to save in main PC folders will be blocked as Bitdefender reads the activity through Krita as malicious for some reason.
*** Bug 384754 has been marked as a duplicate of this bug. ***
Bitdefender is a 3rdparty product, isn't it? Not something that comes with Windows itself. In general, one shouldn't install any 3rdparty "security" software on Windows. All those packages have problems. Krita is safe; you should report this issue to Bitdefender and/or remove bitdefender from your system.
I'm 100% sure Bitdefender is *not* something that is automatically installed for any Windows 10 users. This aside, if some third-party security software decides to block some programs like Krita from writing to files, then there's nothing we can do about that. It really is the users' responsibility to add Krita to the exclusion list (if they know what they're doing).