I’m sorry I can’t provide a better explanation for “after a longer time, the requests work”, because I renew certificates automatically (and cache OCSP responsed locally) and this always happened when I was not sitting at the computer.
At 13:06:28 local time I still got error 6 from the OCSP servers. I see similar errors in my logs until 13:15:13. So, at that time the issue might got solved or not new OCSP-cache requests were performed.