There was some security hardening it seems. I have a local imap server running on my laptop (for interoperability as mail UAs other than Mutt are unable to just eat some mail storage hierarchy). Access is only localhost. After the update to 20260924, the UA wasn’t able to connect to the server anymore. I suspected apparmor, but this wasn’t it. I suspected the firewall, but that wasn’t it. I was able to login, but then the server was not able to lock some logfile and was in an infinite loop while consuming 100% CPU.
The solution was to re-install dovecot from Scratch using the OpenSuseHowTo. This HowTo is outdated as the dovecot devs have changed all the names of all the variables from 2.4.0 to 2.4.5., but I found the updates. Not only did they change ssl_cert to ssl_server_cert_file, but they also require certs and a DH-key. So the initial way of just using localhost only and have no security and just connecting to port 143 does not work anymore. One has to install a production ready server even though just local use is intended.
Funnily enough, the OpenSuse HowTo is not restricting access via the dovecot config, but only via the firewall. But that works so I now collect my local mail via port 993 over an encrypted channel. It works now, but is one of those reasons why people are reluctant to update their system.
This just to inform people who have the same issue.