Bug or missing feature: CARP sync user
It seems that the default user for CARP synchronization is "admin" which is the pfSense default username. There seems to be not way to specify a user name for CARP synchronization in the GUI however, so if the default username is changed from "admin", CARP ceases to work since it always tries "admin" as the username on the remote box.
it now uses whatever username is defined in system -> general -> username.
This actually is a cosmetic bug in the logfiles. It always outputs the user "admin" in the log when it tires to sync though it's using the webgui user (so webguiusernames have to be identical on both systems). Like Scott said, it works and I know for sure as I have been on a cluster with a non default webgui user lately.
It will show up correctly in the next version now but it's a cosmetic issue with current versions. The sync will work.