Error for Group Booking

Currently running:
Events Booking 3.17.0
Joomla 3.10.0
PHP 7.4.16

I know the error was happening on EB 3.16.x. We recently updated to Joomla 3.10. I do not have knowledge of the exact day it broke and what was on what versions. 
I attached the error that pops up. It only happens on Group registrations and does not happen for an individual. 

Has anyone else experienced this recently? 

Hello Robert

Usually, this causes by something on your site which calls addCustomTag method which is not supported in JDocumentRaw (in case Ajax request is used)

I supported one customer yesterday and found that it was System Custom Canonion plugin (I might not remember the name correctly). Could you please check to see if something like that it using on your site? If Yes, please disable it or contact the author of the plugin and ask him to fix the bug

If you cannot figure out, submit a support ticket sending me super admin account of your site and link to registration form so that I can help checking it

Regards,

Tuan