Immune_Hosts option in the .ini file. Allows you to exempt IP/hostmasks from every being added to the ban list and should definitely include the address of your BNC
Interesting that 6.4.3 won't crash for you and newer versions will. It's entirely possible I've introduced a bug in the banning/rejection message rate limiting code that got introduced in 6.5.0+ since a number of changes were made. That's my best guess as to where to look for a race condition so I'll give it a look... Though a number of login related code changes were also made for temporary accounts, deleted but not purged accounts, etc. Perhaps something there isn't happy under stress... It certainly looks and appears to work fine in small setups though.
Appverifier is what I'd start running to find this problem locally, so if you get errors off it, let me know. There are a few which are "ok" though.