Audio streaming over https/TLS fails (TLS certificate verification issue with GnuTLS) [answered]

Sailwave is a 3rd party app, contact the author of the application directly first.

I've already done that, although I thought if the app was offered in the Jolla Store, Jolla would be a little responsible.

It doesn't seem to be the problem of the app, other radio/stream apps from the Jolla Store had the same problem.

But let's wait and see

@olf I generally support your habit of keeping TJC in order but closing this question with an answer was accepted without any accepted answer is ... possibly premature.

Well, as @Bellinianswered his own question, concluding with "The problem has solved itself...", I indeed think that "this question was answered, and the answer accepted", literally.

If you think it should stay open, because the technical background of this issue is not fully understood yet, feel free to reopen it. But that was not the question of the OP (which he answered himself). Hence it might be more appropriate to pose a new question with a broader scope and a test case, which still works (i.e., is still failing).
IMHO this would need a bit more analysis, if a component of SailfishOS fails or the actual app, and if they correctly fail (because of outdated certificates) or because of fetching and parsing an outdated intermediate certificate even though a newer, valid one is offered (this is the current issue, you pointed to). Without this information, the sailors will likely shrug their shoulders, mumbling "some issue with some third party app, so what?". ;)