I do not believe there is anything you can change in your proxy to help AMB establish the subscription.
This is a bug in ServiceNow AMB Client Java code as described by Problem PRB717454.
PRB717454 :: After Helsinki upgrade, "OutOfMemoryError" occurs, and SCCM jobs are not being processed. The fixed will be in Istanbul Patch 1 and Helsinki Patch 7.
You may want to open a ticket with SNOW Support and have them associate it with the problem.
You can also see this issue if the MID Server user has "
