This is a regression bug.
The 0.19 version was not broken.
Bug, IMHO, is due to the incorrect construction of redirect URL in set_project.php.

Concerned code in 0.19 set_project.php, line 52:
$t_redirect_url = 'view_all_set.php?type=3&source_query_id=' . $t_source_filter_id;

In 1.0.0rc2 set_project.php, line 59:
$t_redirect_url = 'view_all_set.php?type=3&source_query_id=' . $t_source_filter_id;

Note the erroneous "&" in 1.0.0rc2, should be "&", instead.

I've run into this problem as well, and I've just verified that the supplied fix by bifit works. Thanks! This bug was bugging me. :)

bifit, this issue is not reproducible with current version of MantisBT.
Feel free to reopen the issue if there are still problems after upgrading.