Core: Exception handler (WEB): Uncaught TYPO3 Exception: Cannot use object of type __PHP_Incomplete_Class as array | Error thrown in file typo3/sysext/backend/Classes/Controller/Page/TreeController.php in line 189
This happened after a core update to TYPO3 - 9.5.17
https://forge.typo3.org/issues/91407
The following thanks to Michael Hitzler.
As far as I can see there is already a solution within the install tool in class BackendUserConfigurationUpdate. This seems to address exactly the issue.
Not quite sure in which version the additional migration task has been added, but it helps you solving the issue system wide.
Just got to module Admin Tools -> Update and select Update Wizard. There you should see a new, not yet executed migragtion task: Update backend user configuration array The backend user "uc" array, which is persisted in the db, now only allows for arrays inside its structure instead of stdClass objects. Update the uc structure for all backend users.
Execute this migration task and your BE users will be updated and have a sane uc configuration in the end.
Problem solved and page tree can be loaeded again.
./typo3cms upgrade:wizard backendUsersConfiguration
Should solve the issue.