I don't think 6.3.1 works either. And I believe it is an issue with 64 bit versions of Sonar (My 8.5.3 does not work with UAD 6.3.1 or 6.3.2)
They made the big change in 6.3.1.
Also, the UAD OCTO requires 6.3.1 or 6.3.2, so no sense in getting an OCTO right now for Sonar 64 bit.
The good news may be that UAD is suppose to release 64 bit software before the end of December and that software MAY be compatible with 64 bit Sonar versions.