No the original issue is still not found. I edited the project 2 weeks ago, and then went on to other tasks. Today was the first time I returned to this project, and the project-file was simply disappeared from the computer. I did a complete search of all drives - also my external - and it was nowhere to be found.
Neither in the trash, so no accidental deletion should be the case.
But thanks again, now I can present the data tomorrow as planned.