Once you have exhusted the cphaprob commands and packet captures have been run for port UDP/8116 all to no avail you may want to run a debug on ClusterXL. The steps are detailed below :

Enable debugging

fw ctl debug -x
fw ctl debug -buf 4096
fw ctl debug -m cluster all
fw ctl kdebug-f > file_name.txt

Disable debugging

[ctrl + c]
fw ctl debug 0