Finally, I got access to a link showing this error. I believe I've cracked it. It seems people copy/paste other answers. If they copy the part of the answer that includes microdata, then the microdata is also copied. So when they paste the content, they also paste the microdata from the original answer.
This results in an answer having the "text" attribute of its own and also a nested "text" attribute from the original answer. This should be fixed by just disallowing all microdata tags.
I've applied a fix and the errors went away in https://search.google.com/test/rich-results. Surprisingly, the validator https://validator.schema.org doesn't show any error at all so you won't see any change there.
The step to fix this is adding this line: https://github.com/q2a/question2answer/pull/909/files. Please, let me know if this gets the issue fixed on your side as well.