While checking the traffic learning suggestion in getting error like "Failed to load suggestion".
I wonder if this is an issue with the ASM database. Is the ASM part of a HA pair? If so if you failover and trigger an illegal request does the traffic learning work on the other device? If so then perhaps reboot the one you've got an issue with?
If not then perhaps this solution will help with the ASM db Troubleshooting the BIG-IP ASM MySQL database
Hope this helps,