That is right... When the feedOp is "Update", Active Sync checks out the User view along with the ActiveSync view. Following need to considered:
1.) Try to be more explicit in determining the feedOp type before applying any attribute values.
2.) Generally this exception is thrown when you are trying to set up the attributes like "waveset.accountId" or "password". Make sure that you are not trying to perform this operations in case of feedOp as "Update"
3.) You will have to provision the User view before even applying the changes to the user. Try using "reprovision" in your WF that is being invoked form activesync.
Cheers
Lalit