I would open a support case on this one. There were some NTLM issues with windows domain membership in the initial 6.9 releases, but they were corrected in the revC release (11282).
To downgrade, this is not supported as there are other packages that are installed along with the kernel. So a "downgrade" would mean reimaging the appliance.
If you open a support case I would be curious to see a tcpdump of the problem along with a feedback. Please do not post this information to the community.