License server error even though you can ping the license server -
This problem may occur in a) certain clients, and b) when one particular system was chosen as the license server.
This is probably caused because of certain configurations of the system's settings, firewall / antivirus which may be blocking the NetSim license check-out within the specific time.
Our suggestion is -
a) That you have two systems where the license server can be run. That way if one PC causes problems you can switch to the other.
b) Note that clients where you face this problem, and turn off the firewall / antivirus while running NetSim. Or if that is not possible to avoid using those PC's when running NetSim