see https://support.atlassian.com/browse/JST-88

Trying to remove an user fails, then appears to have worked later. Pull in the Crowd guys on this, as I'm certain its a Crowd caching issue.

From the ticket:
We are paying for a 5 user license for Jira Studio. I've created 2 users, both of whom will have admin rights. But the Studio Administration page says that we only have two more users left to assign, and offers to upgrade.

We do not need a user called "admin" and have exactly 5 users whom we need to assign access to Jira Studio. When I try to delete this generic admin user (from my username of stu, which has administration rights) I get the following message:

"Exception trying to remove user: com.opensymphony.user.ImmutableException: Credentials provider failed to remove user"

It's fine if atlassian would like this user to be there for their purposes, but not if it counts against the user quota we're paying for. If we're paying per user, and have 5 users to assign, it is not acceptable that in reality we have only 4. This will be a showstopper for us, so I look forward to your response.

Then:
Argh! Now the admin user is gone. It's what I wanted, but it's confusing to me why I got an error message saying the remove failed (which is why I submitted the ticket) and when I re-login it is gone. Np, I'm happy with the outcome. A bit odd though.

Excited about exploring Jira Studio. We've been waiting for this to go live for a while, and it looks like a fantastic product.