Use of `content` field in portal docents must be array.Previously it could be a string but this is no longer supported. was deprecated in MediaWiki 1.35.0. [Called from VectorTemplate::getSkinData in /srv/mediawiki/php-1.35.0-wmf.32/skins/Vector/includes/VectorTemplate.php at line 223]
This relates to a recent change in Vector
About 628 errors in the last 3 days
Looking at the errors i believe this is originating from the Insider extension because that does seem to be adding HTML links instead of data arrays.
To fix, this should be updated to return an array liek so:
[ 'text'=> $userTitle->getText(), "href" => $userTitle->getLocalUrl(), "class" =>'interwiki-insider', ]