> What happened to db1 ? 

It is probably in corrupted state since last outage.

I wasn't fully sure about the plan to fight the outage and decided to try restoring logical backup in pg 12 on mirrordb2, leaving mirrordb1 as is just in case (if restoration fails or problem is somehow mirrordb1-specific and for eventual further troubleshooting of root cause of failures).

Since root cause of the problems is more or less known - the action plan may be to let mirrordb1 replicate mirrordb2 or make mirrordb1 primary again with pg 13, then upgrade mirrordb2 to pg13 and let it replicate mirrordb1.

-- Managing Director: Felix Imendörffer