Thank you very much, it works again. You saved me.
But IMO my case brougth up some elementary problems.
Plesae let me say first that I knew of course that working with a development snapshot is at high and my own risk!
The debug output of JabRef3.8.1 I posted. I couldn't find a hint in it which bib-file it tried to open and that a problem occured while opening it. Maybe I missunderstand that output? If not I would recomment to improve the console/debug output at that point. If you agree I would open an issue for that.
The dev-snapshot reformated the bib-file in that way it wasn't backwards compatible. This is ok but should be done transparent and more communicative!
An extra backup (not the default bak-file) should be done automaticly if a bib-file is transformed that way. Name it e.g.
myfile.bib.3.8.1.bak. Issue for that?
Such a transformation (including the name of the extra backup file) should be clear communicated to the user. A message dialog should be opened to inform that user about the transformation and that there is no way back (except with the extra backup file). Issue for that?