Afaik, IB does not unilaterally block any vpn IPs, and IP blocking is used pretty much as a last resort. That said, if the the IP(s) in question really have been blocked, it's possible it was done in error while dealing with a spam attack or other disciplinary action.

The best course of action is to send a PM to the IB techs, giving the specific IP or IP range that appears to be having access difficulty. If there is an inappropriate block in place, it would be good to get the issue corrected so others don't encounter the same problem.