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I get that error here all the time....
View xoops.org as a guest and click on any forum topic. If you want to reply to a post, you'll have to "Register to Post". Click on that and you are taken to a user login page... Login and you'll get the login re-direct as being successful and then you'll get another re-direct call that should be sending you to a reply editor but says Error - Forum not selected.
A fix for this would be very helpful....