I had also same issue, upgrading from 12.04 to 13.01 completed but the page was down with HTTP 500.
I connected to the server via SSH and executed sudo -u www-data php occ upgrade
inside the Nextcloud folder, solved the problem.
I saw below warnings in the output:
Disabled incompatible app: files_accesscontrol
Disabled incompatible app: files_automatedtagging
Disabled incompatible app: files_clipboard
Disabled incompatible app: files_retention