The Foursquare API now requires a version parameter to be sent (as of 28 Jan) and since the WebOS app doesn't send one, you can no longer login. RIP Foursquare on WebOS.
-- Sent from my Palm Pre3 using Forums
The app author has been real good about supporting webOS, so don't be surprised if we see a fix come our way.
BTW I see you are using the Forums app. You should check out this thread for a patch that fixes a number of issues with the app.
*How to install .patch files on your device*