I know this bug is a couple of years' old and I don't know if anyone other than me still uses Studio 11, but I found a way to fix it! Or well, at least in all the situations where I've used it, it seems to get the job done.
1- Close the error message
2- File>Save As...
3- choose a different location than where the previous SDLXIFF file used to be
4- in that NEW location, also rename the file. Just rename it "1" or "new file" or something short. I think renaming it something short is essential
5- hit SAVE
6- Now save your translation and the problem should now be solved!
I'll be posting this on all the "content null" posts I can find in case it's helpful to anyone else, so if you see it all over Proz, it's not Spam! it's just joy at finally having cracked this pesky S.O.B.!