I just had an issue, after a CAU automatic patch, the AG always On group disappeared on secondary node, it was missing. A quick fix — Restart the cluster service. You can try to add the group back by T-SQL, you can try to add a dummy database and you can suspend one DB and resume. It just a test, this might work. Error log: A connection timeout has occurred on a previously established connection to availability replica ‘Primary Server’ with id [F58F54A2-F919-4BCF-9DB7-CBD2B10981BF]. Either a networking or a firewall issue exists or the availability replica has transitioned to the resolving role. Always On Availability Groups connection with primary database…