Again,
The fact that after it’s done the task for the systems to do get registered, you can register the other hosts, tells me it’s a limitation of connections. Maybe some other limitation is occurring on the server, but something is stopping the connections to allow the registration to happen. I’m fairly sure it’s not a problem in a configuration of the FOG stuff, but I could be wrong.
Also,
Try increasing the connection limit to mysql from 100 to, for testing this, to 999 and see if you run into the same issue.
I say this because sometimes connections can happen that just fail, so the system will create a new connection. 100, when doing enmass of 30, is not very much considering the overall number of connections made.