I don't even see a plugin in the Logo Rotation code that uses the parse_templates hook location - it uses the template_register_var hook location.
Yeah, when I use that it generates a 503 error - When I pointed this out Thundercat (the OP) told me he had similar and used the Parse_Template hook in instead.
I had figured that if that one worked with the forum listings I could duplicate it and try another location for that would work in forumdisplay but sadly that didn't work either.
I take it that you run the w3schools site in your sig? I might need to spend a few weeks studying to catch up with what's going on, tbh