Views exposed filter error "An illegal choice has been detected. Please contact the site administrator."
I was having the same issue. In my case the 'illegal choice' message was displayed when changing the country from 'United States' (the default value) to 'United Kingdom' and selecting 'North Yorkshire' as state/province and saving.

I did the following to fix it:


Go to the Configure page of the Location field (by editing the node type and going to 'Manage fields').
In the 'Events settings' section (not the 'Locative information' section), there is a drop down group called 'Default value'. I set 'Country' to United Kingdom.
Save field settings.
Done.


Using:


Drupal 6.28
MySQL 5.1.72
PHP 5.3.3.7+squeeze17
Location: 6.x-3.3-rc3+6-dev (2013-Oct-25)
GMap Location: 6.x-2.0-beta5+1-dev (2013-Sep-30)


Hope it helps.