Please note its not the maintenance that is the problem here but the fact that your PBX cannot communicate with the activation server.
Try configuring the Network Adapter with the 1st DNS to Google 's DNS ( 8.8.8.8 ). and do an ipconfig /flushdns, (possibly restart of Server)
to confirm that the PBX can reach the ERP to activate, open cmd and type nslookup activation.3cx.com and you should see a reply.