After the forum's cache was cleared using the Support feature via the AdminCP, the entire forum broke and even the AdminCP area.

Here's the only page that shows:

Any ideas what I should do next?

I can't immediately think of why (how) the Support Tool could cause that kind of error. What issues were you trying to rectify when you chose to run it ?

Does the 'front end' work OK as your screenshot is the ACP, I do see it says "guest" as the signed in user however.

I'd suggest you submit a ticket in the Client Area for this so a support technician can take a look and get your community back up and running.