Hi, sorry for the late reply.
The problem sounds a bit strange - you've been using fSekrit for years, and suddenly this error appears, without making any changes to the system? That the problem disappears in safe mode hints to me that the problem is with software, not NTFS permissions, and my guess would have been antivirus software. But you say there has been no changes, and that you even uninstalled Avira.
fSekrit works by copying itself to a temporary location and using that as the "editor" executable, to be able to write to the "document" executable (it's not possible to write to the executable file of a running program without doing some very
dirty tricks). Under normal operation, this means the editor executable is in %TEMP%, unless you have a file called fSekrit.portable in the same folder as the document executable.
When saving, fSekrit starts by writing to a temporary file, then it deletes the document.exe, and finally moves the temporary file to the document.exe name. The fSekrit 1.40 codebase doesn't check for errors deleting document.exe, so that's probably the step that fails.
I'm not sure why that happens, as it's not something I've seen under normal operation (I'm running Win10 these days, but fSekrit 1.40 has been running on pretty much every version from Win9x). But something is probably keeping the document.exe open - this could be backup software, antivirus software, or even explorer.exe. I would suggest grabbing Sysinternals' Handle
and see if it reports some other program keeping the document.exe open.