Nice catch. I also cannot add a new super moderator to my XF2.1 test site with VW4.1.0 Beta 4 b003. And I was planning on (finally) migrating my real site over to XF2.1 in the next 48 hours...

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There are some additional entries in the server error log around the same time:
XF\Db\InvalidQueryException: MySQL statement prepare error [1064]: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1 src/XF/Db/AbstractStatement.php:217
Code:
DELETE FROM `vw_permission` WHERE applytypeid = 0 AND applyid =
------------
#0 src/XF/Db/Mysqli/Statement.php(196): XF\Db\AbstractStatement->getException('MySQL statement...', 1064, '42000')
#1 src/XF/Db/Mysqli/Statement.php(39): XF\Db\Mysqli\Statement->getException('MySQL statement...', 1064, '42000')
#2 src/XF/Db/Mysqli/Statement.php(54): XF\Db\Mysqli\Statement->prepare()
#3 src/XF/Db/AbstractAdapter.php(94): XF\Db\Mysqli\Statement->execute()
#4 src/addons/vw/vw/_core/controller/db/xf2.php(117): XF\Db\AbstractAdapter->query('DELETE FROM `vw...')
#5 src/addons/vw/vw/_core/model/db/mysql/vw.php(1330): vw_DB_Controller_XF2->query_write('DELETE FROM `vw...')
#6 src/addons/vw/vw/_core/model/db/mysql/vw.php(366): vw_DB_MySQL_Model->shutdown_or_run('DELETE FROM `vw...', false)
#7 src/addons/vw/vw/_core/model/plugins/data/vw.php(818): vw_DB_MySQL_Model->delete(Array)
#8 src/addons/vw/vw/XF/Service/User/ContentChange.php(77): vw_Plugins_Data_Model->do_user_cache(Array, Array)
#9 src/XF/MultiPartRunnerTrait.php(48): vw\vw\XF\Service\User\ContentChange->stepCleanupWikiData(NULL, G)
#10 src/XF/Service/User/ContentChange.php(198): XF\Service\User\ContentChange->runLoop(G)
#11 src/XF/Job/UserRenameCleanUp.php(34): XF\Service\User\ContentChange->apply(G)
#12 src/XF/Job/Atomic.php(38): XF\Job\UserRenameCleanUp->run(G)
#13 src/XF/Job/Manager.php(253): XF\Job\Atomic->run(G)
#14 src/XF/Job/Manager.php(195): XF\Job\Manager->runJobInternal(Array, G)
#15 src/XF/Job/Manager.php(111): XF\Job\Manager->runJobEntry(Array, G)
#16 src/XF/Admin/Controller/Tools.php(120): XF\Job\Manager->runByIds(Array, 8)
#17 src/XF/Mvc/Dispatcher.php(321): XF\Admin\Controller\Tools->actionRunJob(Object(XF\Mvc\ParameterBag))
#18 src/XF/Mvc/Dispatcher.php(248): XF\Mvc\Dispatcher->dispatchClass('XF:Tools', 'RunJob', Object(XF\Mvc\RouteMatch), Object(XF\Admin\Controller\Tools), NULL)
#19 src/XF/Mvc/Dispatcher.php(100): XF\Mvc\Dispatcher->dispatchFromMatch(Object(XF\Mvc\RouteMatch), Object(XF\Admin\Controller\Tools), NULL)
#20 src/XF/Mvc/Dispatcher.php(50): XF\Mvc\Dispatcher->dispatchLoop(Object(XF\Mvc\RouteMatch))
#21 src/XF/App.php(2178): XF\Mvc\Dispatcher->run()
#22 src/XF.php(390): XF\App->run()
#23 admin.php(13): XF::runApp('XF\\Admin\\App')
#24 {main}
ErrorException: Template error: reset() expects parameter 1 to be array, null given src/XF/Moderator/AbstractModerator.php:47
Code:
#0 [internal function]: XF\Template\Templater->handleTemplateError(2, 'reset() expects...', '/path/to/forum/...', 47, Array)
#1 src/XF/Moderator/AbstractModerator.php(47): reset(NULL)
#2 src/XF/Entity/ModeratorContent.php(36): XF\Moderator\AbstractModerator->getContentTitle(0)
#3 [internal function]: XF\Entity\ModeratorContent->getContentTitle()
#4 src/XF/Template/Templater.php(999): call_user_func_array(Array, Array)
#5 internal_data/code_cache/templates/l1/s0/admin/moderator_list.php(111): XF\Template\Templater->method(Object(vw\vw\XF\Entity\ModeratorContent), 'getContentTitle', Array)
#6 src/XF/Template/Templater.php(1315): XF\Template\Templater->{closure}(Object(vw\vw\XF\Template\Templater), Array)
#7 src/addons/vw/vw/XF/Template/Templater.php(789): XF\Template\Templater->renderTemplate('moderator_list', Array, true)
#8 src/XF/Template/Template.php(24): vw\vw\XF\Template\Templater->renderTemplate('admin:moderator...', Array)
#9 src/XF/Mvc/Renderer/Html.php(48): XF\Template\Template->render()
#10 src/XF/Mvc/Dispatcher.php(418): XF\Mvc\Renderer\Html->renderView('XF:Moderator\\Li...', 'admin:moderator...', Array)
#11 src/XF/Mvc/Dispatcher.php(400): XF\Mvc\Dispatcher->renderView(Object(XF\Mvc\Renderer\Html), Object(XF\Mvc\Reply\View))
#12 src/XF/Mvc/Dispatcher.php(360): XF\Mvc\Dispatcher->renderReply(Object(XF\Mvc\Renderer\Html), Object(XF\Mvc\Reply\View))
#13 src/XF/Mvc/Dispatcher.php(53): XF\Mvc\Dispatcher->render(Object(XF\Mvc\Reply\View), 'html')
#14 src/XF/App.php(2178): XF\Mvc\Dispatcher->run()
#15 src/XF.php(390): XF\App->run()
#16 admin.php(13): XF::runApp('XF\\Admin\\App')
#17 {main}
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2) I don't experience the error during user removal.