You can view all implemented filter parameters at corresponding section of the guide, for example http://helpcenter.veeam.com/backup/70/r ... ns_id.html
for Backup Task Session.
All other parameters also may work, but it was not specially tested so you can see error message like that one.
If you think that some other parameter for querying should be implemented - here is the best place to tell us about it.
Could you please specify a scenario where you need CreationTimeUTC or EndTimeUTC fields to be queryable?