Try to create a dynagroup with the expression resource.availability = UP when you click OK after filling in the expression, nothing happens - the same page just reloads again. However, other dynagroup expressions will work fine here. I didn't see anything interesting in the server log. This may be related to the linked bug - there may be an error message that isn't showing up.
rev2414 - use case-insensitive value comparison for availability-based group definition expressions;
fixed, rev2424, windows/oracle
This bug was previously known as http://jira.rhq-project.org/browse/RHQ-1266 This bug relates to RHQ-1264