I was having a similar problem suddenly this morning on a client's site. I don't know why it suddenly started, but in my case, it had to do with spaces before and/or after the pipes in the recipient selections. Once I removed those it worked again.
My guess is that perhaps an update made the plugin more picky for syntax on the selections?
How it looked before:
[select choose-recipient "General Contact | info@domain-name.org" "Some Department Name | address@domain-name.org" "Another Person | Joe Citizen <joe@domain-name.org>" "Asst. to Joe | Joe Citizen Jr <joejr@domain-name.org>"]
What fixed it:
[select choose-recipient "General Contact|info@domain-name.org" "Some Department Name|address@domain-name.org" "Another Person|Joe Citizen<joe@domain-name.org>" "Asst. to Joe|Joe Citizen Jr<joejr@domain-name.org>"]