The error reporting from sentence upload is extremely unhelpful: there are so many things that could possibly go wrong, and the error message says nothing about the nature of the problem.
If the problem persists on many repeated attempts, there’s probably a problem with at least one row that doesn’t conform to the rules. If manual inspection doesn’t help, you can try to pinpoint the problem by loading different subsets of the sentences and see which one doesn’t work. By repeating the process you should in the end be able to find the offending entry.
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