Ok, I think I fixed it. Some days ago I changed my entire collection to be visible for friends (it was on "private" before). I just made a quick test with a single entry, whose visibility I changed to "public" and it appeared again on the author page. I changed its visibility to "private" and it still appeared on the author page. So I changed the visibility of my entire collection to "public" and to "friends" again, and everything works again.
Thanks for your support!
When I opened the issue I thought I will be asked for a login, which did not happen. Is there a way now to mark a thread by an aynonymous user as resolved?
I have to bring this up again. After changing the visibility of my collection from "friends" to "public" and back to "friends", now, after several hours, the issue occurs again. Is there any limiation in the visibility of "other -> friends"?