when $wgReadOnly is set, the following message occurs after a login attempt:
[5cbc720cb157233788a1afc7] /index.php/Spezial:PluggableAuthLogin JobQueueReadOnlyError from line 697 of /var/www/html/includes/jobqueue/JobQueue.php: Job queue is read-only: This wiki is currently being upgraded to a newer software version. Please check back in a couple of hours.
Backtrace:
#0 /var/www/html/includes/jobqueue/JobQueue.php(320): JobQueue->assertNotReadOnly()
#1 /var/www/html/includes/jobqueue/JobQueue.php(306): JobQueue->batchPush(array, integer)
#2 /var/www/html/includes/jobqueue/JobQueueGroup.php(160): JobQueue->push(array)
#3 /var/www/html/includes/user/UserGroupMembership.php(162): JobQueueGroup->push(array)
#4 /var/www/html/includes/user/User.php(3668): UserGroupMembership->insert(boolean)
#5 /var/www/html/extensions/SimpleSAMLphp/src/AttributeProcessor/MapGroups.php(34): User->addGroup(string)
#6 /var/www/html/extensions/SimpleSAMLphp/includes/SimpleSAMLphp.php(190): MediaWiki\Extension\SimpleSAMLphp\AttributeProcessor\MapGroups->run()
#7 /var/www/html/includes/Hooks.php(177): SimpleSAMLphp::populateGroups(User)
#8 /var/www/html/includes/Hooks.php(205): Hooks::callHook(string, array, array, NULL)
#9 /var/www/html/extensions/PluggableAuth/includes/PluggableAuthLogin.php(44): Hooks::run(string, array)
#10 /var/www/html/includes/specialpage/SpecialPage.php(565): PluggableAuthLogin->execute(NULL)
#11 /var/www/html/includes/specialpage/SpecialPageFactory.php(568): SpecialPage->run(NULL)
#12 /var/www/html/includes/MediaWiki.php(288): SpecialPageFactory::executePath(Title, RequestContext)
#13 /var/www/html/includes/MediaWiki.php(861): MediaWiki->performRequest()
#14 /var/www/html/includes/MediaWiki.php(524): MediaWiki->main()
#15 /var/www/html/index.php(42): MediaWiki->run()
#16 {main}
The message is expected to occur in order to prevent any login attempt, but the format looks more like rather an error than a message and produces confusion.