Though my current QGIS-project was not saved in that very moment, yesterday evening my Win 10 decided to run an OS-update and shutted down. 'No problem', I thought because of having installed the AutoSaver plugin, which has created the bak-file as exspected.
According to several forums just renaming the file suffix from yourfilename.qgz.bak to yourfilename.qgs should be sufficient. But, trying to load the re-suffixed file into QGIS resulted in kind of a blank project (though showing the right filename in the headline of the QGIS-Window). Comparing the autosave-bak-file with an earlier qgs-file (created by 'normal' saving) in the Editor didn't reveal any file error or corruption. So, everything seemed to be allright with the autosaved file.
I was struggling around quite a long time, not finding any answer in the web. (There is a similar thread on stackexchange: QGIS AutoSave Plugins File not Recovering. But this helped not to solve my specific issue).
As I already found the solution, I will give the answer by myself. It's not very spectacular, but maybe someone else with having the same problem will save some time and nerves.