QGIS 3.16 with AutoSaver plugin: Renamed bak-files were not opened correctly or just as a blank project, respectively. (Win 10)

Question

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.

Answer 1

Let me say first of all: The problem might only occur with Windows, even only with Win 10. I cannot judge this for other OS.

The solution - in my case - was not simply to rename the file in the Win-Explorer, but open the AutoSave-bak-file in the Win-Editor (any other editor would do it as well, I guess) and then save it by 'save as ...' (with precautionarely a new filename and) with qgs-suffix (not with txt of course).
Afterwards opening the file in QGis worked as usual, without any problems.

I am not an expert, but background seems to be: Just renaming the suffix with the Explorer will not neccessarely change the file-type information (Explorer --> right-klick on file --> properties). In my case at least, despite having renamed the file to .qgs the properties dialog showed the wrong file type: Either .qgz, .bak or .qgz.bak, - I don't remember exactly. But anyway not .qgs. So, I guess, this wrong information leads QGIS to read the file the wrong way.
However, the little Editor-detour solved this problem.

Little note for AutoSaver-filetype-beginners: Please be aware to rename the file from yourfilename.qgz.bak to yourfilename.qgs (not to yourfilename.qgz) as the bak-file is based on the qgs-format no matter which format you are basically using. This is also the reason, why the AutoSave-file is 10 or 50 times larger than the origin qgz version.