I received an "Illegal choice field_event_icalday in Date field to use with signup element." message in my admin/reports/event/364
In this case field_event_icalday was a content field created to select a date with the javascript popup calendar

So if signup is enabled on a content type and then exported it can't be imported anymore

.. actually when you have previously exported a content type with signup module enabled on that content type you can't import it anymore.. what you need to do apparently is to delete the signup elements in that export file (text file) and then it imports flawlessly and then you can enable it again.

I hope this info helps the maintainer to enhance the code of signup coz this is a real cool module!

BassPlaya