I had this problem 2 years ago and it ended getting fixed. Recently I seen it again and the problem was that my container running synapse did not have working D

I had this problem 2 years ago and it ended getting fixed. Recently I seen it again and the problem was that my container running synapse did not have working DNS which caused all notifications to stop working. Once I fixed up resolv.conf all was working. I am not suggesting that was your issue but it might help somebody.