The hub can only handle a maximum of 64 subscribers in a Windows environment due to Windows OS (winsock) limitations on the number of open sockets. As a result, you may see probes have their socket connections dropped or reset. This applies to all probes, both core and monitoring probes. The number of subscribers for Linux/Unix hubs is only limited by available resources.
Normally the recommendation to resolve this issue is to off-load these to a "sibling" Unix hub. Unix systems do not have this limitation. You can also change the Get/Attach queues to POST and this will not count towards the subscriber limit. First step is to offload any monitoring probes or probes and their associated queues to another, e.g., secondary hub. Also note that UIM admin logins to IM count as a subscriber/temp subscribers so perhaps some admins can login to other hubs.