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.