The prefix option for Parsoid is documented as optional, but when it isn't set the Visual Editor breaks in an esoteric way. When prefix isn't set and you edit an article to have an external link, in some cases it will not save the article and give the message "unknown error". The problem seems to be in the Collection extension, assuming that prefix is set.
This error is generated:
NOTICE: PHP message: PHP Notice: Undefined index: prefix in /.../mediawiki-1.32.0/extensions/Collection/rendering/CollectionRenderingAPI.php on line 250