Assuming that you are using WCS with the lobby ambassador account to create guest users and not a super admin or admin account, which is not supported, check the following:
-Make sure in WCS there is an SNMP RW community string configured as well as on the WLC.
-Increase the max local database entries under WLC-Security-AAA settings to 1024.
IF that does not solve the issue, please set the logging to Trace in Administration -> Logging , recreate the problem and post the logs.zip file.
Also provide your versions of WCS and WLC.