After an upgrade from 5.3.3.2 (the last 5.3.x version) to 5.4.0.5 - I find that editing content or trying to add new content blocks to pages is broken.

I created a new page, put it in edit mode and went to add a new block (block type doesn't seem to matter) and as soon as I add it I get an error:

{"aID":"228","arHandle":"Sidebar","cID":"120","error":false,"bID":"453"}

(see attached screenshot)

I have deleted the caches, logged out and in, and the content does seem to appear there but it's a fight to get it there in the first place. When I click Exit Edit Mode the little ring of balls just sits there and spins and it never exits. I have to log out, log back in and approve my edit and then the content shows up.

Can anyone point me in the right direction?

Thanks,
Joe