Unable to display category or tag forms

I cannot display any tag or category forms. I’ve completed some troubleshooting my side, and categories.php is being called, but as soon as a category itself is selected, form categories-question-list.php does not fire. I cannot for the life of me think why. I am running the DWQA plugin also, which works absolutely fine (AnsPress and DWQA are very similar).

I do not think this is theme related because DWQA works fine – the theme vendor also informs me that they cannot see the reason as to why the form will not load, and have pushed this back on AnsPress. Seriously, I really want to use this plugin, but without categories and tags working properly, it isn’t going to be possible !

Surely, someone must have an idea as to why this is ? Switching themes resolves the problem, but having paid a premium for an extremely popular and well supported theme, I clearly do not want to go down this route. Am I the only one using the X-Pro theme ??

Thanks (for any help anyone can give !)