If after running fwaccel off you find that whatever issue you have is solved, instead of turning of SecureXL for all traffic permanently via cpconfig, consider excluding the problematic IP address(es) from acceleration as described here:
sk104468: How to disable SecureXL for specific IP addresses
After defining this exclusion everything matching it will always go F2F/slowpath, which has the side effect of making the matched traffic fully visible if using fw monitor.
Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones