Steps to reproduce:
For a week now it fails with the traceback below.
Either config problem at wmflabs deployment or special page code modified.
Recent changes (watchlist) not affected.
Exception encountered, of type "Wikimedia\Assert\ParameterTypeException" [V01xrApEEaoAAGxX1zoAAAAB] /wiki/Spezial:Beobachtungsliste_bearbeiten Wikimedia\Assert\ParameterTypeException from line 89 of /srv/mediawiki/php-master/vendor/wikimedia/assert/src/Assert.php: Bad value for parameter $dbkey: must be a string Backtrace: #0 /srv/mediawiki/php-master/includes/title/TitleValue.php(76): Wikimedia\Assert\Assert::parameterType(string, integer, string) #1 /srv/mediawiki/php-master/includes/cache/LinkBatch.php(171): TitleValue->__construct(integer, integer) #2 /srv/mediawiki/php-master/includes/cache/LinkBatch.php(135): LinkBatch->addResultToCache(LinkCache, ResultWrapper) #3 /srv/mediawiki/php-master/includes/cache/LinkBatch.php(122): LinkBatch->executeInto(LinkCache) #4 /srv/mediawiki/php-master/includes/specials/SpecialEditWatchlist.php(385): LinkBatch->execute() #5 /srv/mediawiki/php-master/includes/specials/SpecialEditWatchlist.php(541): SpecialEditWatchlist->getWatchlistInfo() #6 /srv/mediawiki/php-master/includes/specials/SpecialEditWatchlist.php(151): SpecialEditWatchlist->getNormalForm() #7 /srv/mediawiki/php-master/includes/specials/SpecialEditWatchlist.php(130): SpecialEditWatchlist->executeViewEditWatchlist() #8 /srv/mediawiki/php-master/includes/specialpage/SpecialPage.php(479): SpecialEditWatchlist->execute(boolean) #9 /srv/mediawiki/php-master/includes/specialpage/SpecialPageFactory.php(591): SpecialPage->run(NULL) #10 /srv/mediawiki/php-master/includes/MediaWiki.php(282): SpecialPageFactory::executePath(Title, RequestContext) #11 /srv/mediawiki/php-master/includes/MediaWiki.php(746): MediaWiki->performRequest() #12 /srv/mediawiki/php-master/includes/MediaWiki.php(520): MediaWiki->main() #13 /srv/mediawiki/php-master/index.php(43): MediaWiki->run() #14 /srv/mediawiki/w/index.php(3): include(string) #15 {main}