Can't configure CSS properties on mini panel content

I've set up a simple mini panel with 2 columns, each containing one row with one region each. I've added 'custom content' to it. However when I click on the cogwheel and chose the 'CSS properties' option I get a JS alert popup containing the message:

An error occurred while attempting to process /panels/ajax/pane-css/5/24

Before the error message, the overlay does appear containing no form fields and just the loading icon. The overlay content never loads. This doesn't happen when I try doing the same thing on panel pages.

Cheers,
Stella