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.
https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/Insider/+/master/includes/InsiderHooks.php#118
https://logstash.wikimedia.org/goto/21b810acf89c9d8dc6bd5f9fa8cd8422
To fix, this should be updated to return an array liek so:
[ 'text'=> $userTitle->getText(), "href" => $userTitle->getLocalUrl(), "class" =>'interwiki-insider', ]