Start with switching off the validation in your project settings:

This should already help.

The cause is typically merging segments over paragraph breaks.

I merged some segments but I don't think I have merged across paragraphs.

The error message points to this fragment:

I assume the mid="68" fragment means there is something wrong with the segment No 68. However, I do not have segment 68 - it has been probably merged or split. (I have: 66a, 66ba, 66bba, 66bbb, and then 71.)

Thank you for your answer Jerzy Czopik. I have switched the validation off as you suggested, and I do not get the error message any more. However, is it safe to work with the validation off? And is there a way to fix the error in the XLIFF file?.  

I am not sure if we unexperienced user can repair the sdlxliff. However, my experience so far shows no problem with validation switched off.

And the origin indeed points to a merged paragraph. So this is certainly the cause. Some tags inside can also cause problems.

Thanks again, Jerzy Czopik.

Should it be considered a Studio bug? The segments have been merged / split in the Studio, the XLIFF file was not edited externally, nor the corresponding source and target language files. I assume Studio has incorrectly saved the information about merged / split segments and the Studio itself reports the error. Am I right in thinking in this way?

Regarding the repairing the sdlxliff file, I rather thought about a "repair" function in or outside the Studio.

I don't think it is a bug. This happens only very rarely. And as you can continue working and save target, no harm is done. I would simply forget it this time.

I don't think it is a bug. This happens only very rarely. And as you can continue working and save target, no harm is done. I would simply forget it this time.

OK, I will do so. However, in case anyone from the team wants to look at this, I am ready to help and provide required information (if the above is not sufficient).