We try to parse this response as JSON and it fails due to an unexpected token error (the first : specifically).
I have no idea what that CSS is or why it is trying to insert it into AJAX responses. That CSS is nothing to do with XF.
I'm not even sure it's possible for us to properly workaround this so realistically the best solution is to configure uBlock with an exception. I don't know specifically which exception but certainly it's possible to prevent it with this option:
(The eye icon) click that and it switches off cosmetic filtering for this site only.