I think the update step 5251 was added in ILIAS 5.4 for migrate org unit default positions (which may exists always no matter using it)
https://github.com/ILIAS-eLearning/ILIAS/commit/f8234d1b970e3a69d2494201e2742458f116231b
The event is triggered when assign user to org unit
But somehow the event raiser seems not more defined in the update context
The fix will define it if not defined already (Untested if it fix the problem)
ILIAS 5.4 https://github.com/ILIAS-eLearning/ILIAS/pull/3367
ILIAS 6 https://github.com/ILIAS-eLearning/ILIAS/pull/3365
ILIAS 7 https://github.com/ILIAS-eLearning/ILIAS/pull/3364
ILIAS 8 https://github.com/ILIAS-eLearning/ILIAS/pull/3366
But after modifying the code as explained, I have a new problem when running the migration operation...
An error appears:
[Tue May 04 11:05:13.645955 2021] [php7:error] [pid 38364] [client 160.32.xxxx.16xxxx4:xxxxx] PHP Fatal error: Uncaught InvalidArgumentException: Identifier "ilPluginAdmin" is not defined. in /var/www/html/ilias/libs/composer/vendor/pimple/pimple/src/Pimple/Container.php:96\nStack trace:\n#0 /var/www/html/ilias/Services/Component/classes/class.ilPlugin.php(1239): Pimple\\Container->offsetGet()\n#1 /var/www/html/ilias/Services/Component/classes/class.ilPluginAdmin.php(423): ilPlugin::getActivePluginsForSlot()\n#2 /var/www/html/ilias/Services/EventHandling/classes/class.ilAppEventHandler.php(197): ilPluginAdmin::getActivePluginsForSlot()\n#3 /var/www/html/ilias/Modules/OrgUnit/classes/Positions/UserAssignment/class.ilOrgUnitUserAssignment.php(90): ilAppEventHandler->raise()\n#4 /var/www/html/ilias/Modules/OrgUnit/classes/Positions/UserAssignment/class.ilOrgUnitUserAssignment.php(96): ilOrgUnitUserAssignment->raiseEvent()\n#5 /var/www/html/ilias/Modules/OrgUnit/classes/Positions/UserAssignment/class.ilOrgUnitUserAssignment.php(77): ilOrgUnitUserAssignment->create()\n#6 /var/www/html/ilias/Services/Database/classes/class.ilDBUpdat in /var/www/html/ilias/libs/composer/vendor/pimple/pimple/src/Pimple/Container.php on line 96, referer: https://xxxxxxxxxx.fr/setup/setup.php?cmd=db&client_id=eadterre&back=clientlist