ADDITIONAL NOTE: After continuing to edit my files (after restarting Geany) for maybe 15 minutes, the error occured again. It is not possible to save the file.
Unfortunately, this version of Geany is not usable for me. I will therefore switch to a different editor, BUTif one of the Geany maintainers wants me to try out something, I will of course turn back to Geany, because I want to help to find the bug.