That's an FTP error returned from your hosting service that BI is just passing along. It's not actually the number of hosts, it's the number of simultaneous connections from whatever number of clients by the same user (even if just one).
You've exceeded and/or not dropped FTP connections you're making within whatever period of time toward that limit either on the BI or hosting side. Maybe creating too many within some period of time by having multiple triggers/cameras trying within the same time frame. The second site may not be making as many attempts to connect based on different numbers of cameras/triggered events/etc.
Under Alerts > Actions: On Alert button > Action set, take a look at how the the FTP server that you've defined is set up. There are settings for timeout and number of connections (90 seconds and 5 connections by default).
If it's a limit on the hosting side, which it may well be, then maybe you can up the limit on your FTP server in your control panel or maybe by user.
