How the issue manifests
When working with a company (e.g., after restoring a backup), the following error appears:
HTTP ERROR 500: OldAppServerException
Cause
The company database was created or updated on a newer version of ABRA Flexi than the version of the server on which you are currently running it.
👉 The server is unable to work with the newer data structure.
Solution
update ABRA Flexi on the server to the same or higher version than the one on which the backup was created
then perform the backup restoration again (if necessary)