File Downloads Suite - cannot Add File Download Plan

File Downloads Suite won't let me add new file download plans.

The path:
/admin/structure/ms_files/add

bypasses the site theme entirely & won't let me actually add a new file download plan. This used to work just fine, but suddenly no longer does, and I haven't done any modifications that would have affected the moneysuite module files or directory ever since I originally installed the module on September 28, 2018.

I've also cleared all caches, & updated all other modules but it still gives me a theme-less page with no ability to actually add a new file (screenshot). Alternately, it just returns a 500 error.