When I try to add a new Task Repository with Redmine 1.2.0 and Mylyn 2.7.2 it works. Task are being shown and Queries works as well. The problem appears when I try to submit a new or change an existing task. After a click on Submit button in a task, message dialog with error message "Submit failed: Server Error " always appears.
I found at https://sourceforge.net/apps/wordpress/redmin-mylyncon/2011/02/20/eclipse-3-5-and-redmine-1-1-now-also-supported/#comment-18 that it could be related with a JSON decoding bug.