The fact that font-lock no longer works has indeed to do with the error
message you get.
The variable org-xemacs-p used to be defined in org.el, but no longer
is. The entire file
does not contain any reference to that variable anymore. The fact that
you this
message means either that you are still loading some old version of
org.el, or that
you are using org-xemacs-p in some configuration, hook, or whatever.
What is the value of C-c h org-version RET ?
If it is not 4.19a, then you are loading some old version of org-mode.
If it is 4.19a, it must be a hook or whatever.
Find any uses of org-xemacs-p, and replace them with (featurep 'xemacs).