Symptoms

Adding a new user to Confluence returns a stack trace:

com.atlassian.core.exception.InfrastructureException: com.atlassian.user.EntityException: Was unable to create user [zedyap] 
but the credentials provider [bucket.user.providers.CachingCredentialsProvider@10e7264] did not say why.

Also, public sign up fails due to this error.

Cause

Confluence user management is delegated to JIRA logins. Confluence has read-only access to JIRA database, hence, the user addition fails.

There is a request at CONF-11917 to fix the misleading error message.

Resolution

Add the new user in JIRA instead. To avoid the misleading error message:

• Turn on the External User Management option. Note that, however, there is a bug in version 2.9.3 onwards that does not prevent users and groups creation.
• Turn off public signup.