Have you merged any segments? Other reason can be missing tag. Translating with tag verifier off is a very bad idea...

You can switch off the validation of SDLXLIFF in your project settings. This will most probably do no harm.

I was translating with the option ON, but switched it OFF to get rid of this error. And I was using single files while translating, no I open all of them as a multiple for the final review. The error show up when  opening the multiple as well as in a single file. 

I was translating with the option ON, but switched it OFF to get rid of this error. And I was using single files while translating, no I open all of them as a multiple for the final review. The error show up when  opening the multiple as well as in a single file. 

Try switching the validation off.

There are many reasons for this error. You can open the sdlxliff in a decent text editor and go to the indicated position to find out, what is causing it.

I will try that, thank you!

Interestingly, the error message points out to the line number 685, which does not exist in this .sdlxliff file! The document ends at line number 683, as checked both in the PSPad and Kate text editors.

I still get the error message with all the verifications switched off in the "options -> verification" :-(