9.04 Great member API features...but

The problems that you mention all sound like they are related to changing the username.

If you change the username of a member in the forum then the forum doesn't see then as a user anymore even though their session is still running. Clearing the cookies deletes the session so that user can be picked up again from the API.

If this is also coursing issues when you just update the members user group then check that the forum and your own site are both running under the same page encoding. If not updating the user group from the admin are could also be updating the username and storing it under a different page encoding, thus the forum can't match up the user again coursing the same problem as just changing the username.

Disabling being able to update the username when the API is enabled should prevent both these problems.