Several add-ons I have user picker dialogs in the dashboard portion in order to assign owners/users to things. This works great, but for some unexplained reason this functionality is now broken on my site. The user picker dialog popup will load up fine and show the first page of users - but if you try and click the next page, search by name, filter/sort, etc it takes you to another page with the results minus the dashboard formatting - basically missing the header/footer - and is nonfunctional at that point.

I know it's not specific to a particular add-on since it's happening with all of them - and the really strange part is that my site is using a vanilla c5 core, which is shared by other sites (multi-site setup) and the user pickers on all of the other sites work fine. I'm thinking there's some trick I'm missing that might fix this - I've tried clearing cache, making sure I have no overrides, etc - still no luck.

Any help would be appreciated!