"Skinr settings weren't saved due to an error."

I've got a site that uses a sub-theme of Fusion (and thus skinr for additional theming).

Whenever I try to create a block with a Block Description and have text in the Block Body I get the error "Skinr settings weren't saved due to an error." and indeed, the skinr settings are not saved. However, if I create a block with only a Block Description initially, or when I edit an existing block to add text to the Block Body, the skinr settings are saved without issue.

Could someone shed some light on exactly what this error means so that I can try to chase down the problem?