$GLOBALS['BE_USER']->uc sometimes wrongly written, gets a stdClass where an array would be expected
It appears that since one of the latest patches (probalby 9.5.9), the uc is sometimes wrongly written and gets an stdClass where an array would be expected. This later leads to fatal errors because php expects an array, but gets an stdClass.
Both issues seem to be related to the pagetree / States and for now just fix the symptoms.
In #88984 it is the following setting that suddenly is an stdClass: