I tried an upgrade from 2.4.24 to 2.5.3 on a staging site ( https://kigges.hwolf.de/ )
First issue with the db update script was a duplicated id in the userdata.useremail colum. I fixed this by deleting this user
After this the upgrade script was ok, but now all date strings a strange like this:
Forumszeit: %02.%08.%2022, %12:%Aug (Europe/Berlin)
Allgemeine Termine - holger, %09.%11.%2018, %13:%Nov […] [*] [○] [x] (Rund um Fußball)
It's with also with the default theme.