Hi Steve,
As per the problem and observation by you in your network it can that one of the desktops / devices got infected with some malware and it is spoofing the address of the LAN and attacking on the server that why you are getting ip address and are getting blocked in server due antivirus installed in the server.
In order to resolve this i would recommend you to scan the desktops with latest antivirus patch.
Hope that clear out your query,If commenst was helpsful do rate the post.
Regards
Ganesh.H