there is quick win for you with this issue. It is likely that you do not need to have Dotmailer module enabled in your site.
So, unless that is the case for you, you may approach this in 2 phases:
disable the modules:
php bin/magento mod:dis Dotdigitalgroup_Email Dotdigitalgroup_Chat
if it fixes the issue and if you genuinely don't need the modules (meaning, you don't have an active account with them), then you may remove the dotmailer related entries from your
composer.json file and then run
composer install. Before doing this, it is important to have a backup of database and site's codebase.
Finally, if you'd want to understand the issue. The place to start is to find in your codebase
app/code folder and
vendor folder places that use the class
XSLTProcessor. Since this word is quite unique, you are likely to find a plugin somewhere in your codebase that needs tuning.