This error could be from connections to the web GUI itself, or could be firewall related. I would typically check the output of "show users" to determine if there are many connections to the unit. Excessive connections to the unit could be the result if there is a malicious attack against the unit or valid connections are not properly terminated. If the sessions are firewall related, the first thing to check is to make sure that the firewall idle timeouts are not set unnecessarily high. You can check firewall session counts using "show ip policy-stats". Feel free to respond with your unit's configuration and the output of the two commands above when you are getting the error message.
I went ahead and marked this as assumed answered, but feel free to ask further questions or provide an update.