After an upgrade to Hogfather there are often problems with DokuWikis that are no longer reachable. Often this is caused by plugins or templates that have to be adapted.
Besides the possibility to deactivate these plugins manually by server intervention, there is also the DokuWiki Recovery Script. This script does some requirement checks and resets the DokuWiki as far as possible back to the default state.
From there, individual plugins can be updated (if available) and reactivated via the Extension Manager and tested step by step.
If the DokuWiki is still not reachable after using the Recovery Script, the installation bug is located in the core installation of DokuWiki. This shouldn't actually happen when upgrading via the Upgrade Plugin.
A previous backup is of course always recommended.
HTH - Michael Sy.