Not sure if this is the right place to ask the question, but...
I've got the following xliff file:
<?xml version="1.0" encoding="UTF-8"?>
<xliff xmlns="urn:oasis:names:tc:xliff:document:1.2" version="1.2">
<file original="file1.txt" datatype="plaintext" source-language="de-de" target-language="en-us">
<body>
<trans-unit id="unit_5">
<source xml:lang="de">unit_5</source>
</trans-unit>
</body>
</file>
</xliff>
I try to open the document as "translate single document" and get the following error from the image below. What might it be?.
I just opened your xliff file in Trados Studio 2014 (SP1) without any error messages coming up. I am afraid I do not really understand what Studio was complaining about for you: the file looks fine to me.
Did you try to disable schema validation in the file type settings for XLIFF, just as a workaround?
Best, Jenszcz