That's really strange, because update 1276 always returns successful - even if something went wrong.
What this update does is add the ed25519 public keys for all channels on your site to the xchan_epubkey column that was added to the xchan table in update 1275. Then it returns success.
There is no standard answer because database updates are by definition unusual events.
In this case, look to see if you have a column called xchan_epubkey in your xchan table. It will probably be at the end. If it isn't there, visit admin/dbsync/1275 which should re-run that update.
Then do a quick search of the xchan_addr column of the xchan table for your webfinger address (einer@digitalesparadies.de) and see that there's some gibberish starting with z6Mk in the xchan_epubkey field. If it's there, all good. If not, visit admin/dbsync/1276 and re-run that update.
I recall we had some problems with 1275 on postgres initially, but that should be fixed.