Okay, I found two other issue: When I report a sentence, e.g. because of spelling mistakes, I get another sentence to validate.
If there are no more unvalidated sentences the field with the reported sentence turns white and stays. I can reload to get rid of it or play it again and press “No” (or "yes, but that’s kinda senseless…)
If I reload it or if there are some unvalidated sentences left I can go on validating - but the reported sentence returns!? Some days ago I had to tell four times that the sentence is bullshit - and finally pressed “no” to get rid of it…