Saving file produces unknow file format

users
#1

Good afternoon (again)
One other question. Maybe related to the one I just posted.

I noticed the other day, that when I use files in a server based file synchronization environment (ownCloud),
.odt files may be transformed immediately or after having been edited a few times into an unknown file format.

Usually, I expect to see something like "file.odt" in file explorer, but for some files - I have no idea why this happens only to SOME files -
I suddenly get just "file".
When I click that file to open it, the computer shows me a list of possibly usuful application to choose from to open that file.

Even creating a new/blank file, copying the text of the original file into a text editor and then from there into the new document
produces the same result.

Does anybody know WHY this happens - and if possible could tell me how to fix it?

Is THIS problem and the other one I described in my previous post possibly another Ver. 7.xxx bug?
-> I have never seen anything like this before.

Thank you.
Thomas