It is not theme specific function.
@kriskoykit means TinyMCE is with the Osclass Script it's not in the theme folder or it's not part of the Theme, TinyMCE is with OSclass and you can enable it at Oc-admin ( listing tab under settings ) then check this option Enable TinyMCE on add/edit listing form in front/back officeHope this helps you.
Do you have error log to that 500 error?
Does not make sense as error 500 should output something to logs, if logging is enabled.I have tested on dev & demo sites and it works, so it could be related to some of plugin functions