@duncanc You nailed it, thanks!
I went to my phplist, logged in, clicked “Config” -> “Subscribe pages” -> “Edit” (icon) -> “Select the lists to offer” tab, and–indeed–there was nothing checked!
Wow, this is a very, very obscure configuration error. I think, at the least, we should add a line to emit an error (exception?) to the logs indicating that an unexpected input was encountered, and that the admin should verify that there is at least one list selected in the relevant Subscribe Page. For example:
ERROR: Unexpected input error: No "lists to offer" selected. Please confirm that at least one list was checked in the "Select the lists to offer" tab of the "Subscribe Pages" config in the phplist admin web interface for Subscribe Page #2 = "Subscribe to our newsletter"