Start with switching off the validation in your project settings:

This should already help.

The cause is typically merging segments over paragraph breaks.

I have merged some segments, but I not think I have merged across paragraphs. The error message points to this fragment of the XLIFF file: "<mrk mtype="x-sdl-location" mid="63f9c784-94e0-413f-801f-460041e5f2ab"/>" and the broader fragment is as follows:

<group><sdl:cxts><sdl:cxt id="26"/><sdl:cxt id="9"/></sdl:cxts><trans-unit id="fc5d03d5-5854-4cf8-8b0b-139c5c6e535a"><source><g id="2795"/><mrk mtype="x-sdl-location" mid="325f6c46-f90c-4396-9420-59c8a6574c8c"/></source><seg-source><g id="2795"/><mrk mtype="seg" mid="68"/><mrk mtype="x-sdl-location" mid="325f6c46-f90c-4396-9420-59c8a6574c8c"/></seg-source><target><g id="2795"/><mrk mtype="seg" mid="68"/><mrk mtype="x-sdl-location" mid="63f9c784-94e0-413f-801f-460041e5f2ab"/></target><sdl:seg-defs><sdl:seg id="68" locked="true" conf="Translated" origin="interactive" origin-system="MergedParagraph"><sdl:value key="SegmentIdentityHash">PrQWIj6eaea7juGXk5Ea0a0gJ9g=</sdl:value></sdl:seg></sdl:seg-defs></trans-unit></group>

I assume that the 'mid="68"' means the segment number 68. There is no segment number 68, it has been merged probably (I have 66a, 66ba, 66bba, 66bbb and then 71).

I have switched the validation off as you suggested, Jerzy Czopik. Thank you for your suggestion. The error is not reported any more. However, is it safe to work with the validation off? Is it a way to fix the error?

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.

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).

Hello, I came across this message and I thank you because uncheking "Validate SDL XLIFF documents before reading" solved my problem!