DOMDocument::loadHTML(): Tag af-field invalid in Entity when loading dashboard
I'm not sure if this is new or not. This might be something coming up for me now with php8. The error comes up in phpquery which is now provided by vendor/rubaboquery which is new, but I'm not sure it's from any differences there. I think the error is technically legit.
What's happening is during hook_civicrm_angularModules, afform loads all the blocks, such as this one: https://github.com/civicrm/civicrm-core/blob/6fa8c627a2f406329370f2b03d656dbc565ccb9f/ext/afform/core/ang/afblockContactAddress.aff.html
At https://github.com/civicrm/civicrm-core/blob/6fa8c627a2f406329370f2b03d656dbc565ccb9f/ext/afform/core/Civi/Afform/Symbols.php#L38, it creates a DOMDocumentWrapper around it, which is provided by rubaboquery. Note that it considers the block html (as opposed to e.g. xml), so calls DOMDocument->loadHTML. So I believe what's happening is that loadHTML validates it against the html schema, and <af-field>
is not a real html tag.
See also https://www.php.net/manual/en/domdocument.loadhtml.php#125526, although I'm not saying that is the best solution. I don't know at the moment.