Apparently the extension is updating tables on load.php requests. This is very bad, and can cause database contention (or deadlocks in this case).
From Topic: DB Error on Login:
[Bug56269] Exception thrown with an uncommited database transaction: [6f314852] /load.php?debug=false&lang=de&modules=site&only=styles&skin=monobook&* DBQueryError from line 1246 of /var/www/mediawiki-1.25.1/includes/db/Database.php: A database error has occurred. Did you forget to run maintenance/update.php after upgrading? See: https://www.mediawiki.org/wiki/Manual:Upgrading#Run_the_update_script
Query: UPDATE `ldap_domains` SET domain = 'domain.local' WHERE user_id = '99'
Function: LdapAuthenticationPlugin::saveDomain
Error: 1213 Deadlock found when trying to get lock; try restarting transaction (localhost)
#0 /var/www/mediawiki-1.25.1/includes/db/Database.php(1205): DatabaseBase->reportQueryError('Deadlock found ...', 1213, 'UPDATE `ldap_d...', 'LdapAuthenticat...', false)
#1 /var/www/mediawiki-1.25.1/includes/db/Database.php(2153): DatabaseBase->query('UPDATE `ldap_d...', 'LdapAuthenticat...')
#2 /var/www/mediawiki-1.25.1/extensions/LdapAuthentication/LdapAuthentication.php(2069): DatabaseBase->update('ldap_domains', Array, Array, 'LdapAuthenticat...')
#3 /var/www/mediawiki-1.25.1/extensions/LdapAuthentication/LdapAuthentication.php(1240): LdapAuthenticationPlugin::saveDomain(Object(User), 'domain.local')
#4 /var/www/mediawiki-1.25.1/extensions/LdapAuthentication/LdapAutoAuthentication.php(63): LdapAuthenticationPlugin->updateUser(Object(User))
#5 [internal function]: LdapAutoAuthentication::Authenticate(Object(User))
#6 /var/www/mediawiki-1.25.1/includes/Hooks.php(209): call_user_func_array('LdapAutoAuthent...', Array)
#7 /var/www/mediawiki-1.25.1/includes/User.php(365): Hooks::run('UserLoadAfterLo...', Array)
#8 /var/www/mediawiki-1.25.1/includes/User.php(2583): User->load()
#9 /var/www/mediawiki-1.25.1/extensions/FormMailer/FormMailer.php(54): User->getRealName()
#10 [internal function]: wfSetupFormMailer()
#11 /var/www/mediawiki-1.25.1/includes/Setup.php(678): call_user_func('wfSetupFormMail...')
#12 /var/www/mediawiki-1.25.1/includes/WebStart.php(138): require_once('/var/www/mediaw...')
#13 /var/www/mediawiki-1.25.1/load.php(30): require('/var/www/mediaw...')
#14 {main}