Transaction log still full after backup
Explanation to this problem was that replication halted on one database in Availability group. Transaction log backup was successful but log file could not be emptied because  replication to secondary server was paused. I did not notice this looking at the databases initially. And didn't notice that log_reuse_wait_desc was showing AVAILABILITY_REPLICA on the primary server (looking at the wrong line in result). But when checking again after Shankys reply I discovered that and Availability group Dashboard showed that replication had halted. Resumed replication, backed up log again and then I was able to shrink logfile to a more accurate size.