Hi guys,
We use UserManager to authenticate via MAC and set queues for DHCP Servers in our network. We just upgraded several hundreds of our Mikrotiks in remote areas from 6.42.x to 6.45.5 and we started to receive thousands of calls complaining about no service. We checked the logs and find out that the server radius connection stopped working, we receive "RADIUS server is not responding" message. I upgraded the usermanager machine to the latest version (also updated the userma package to 6.45.5) and nothing... Any idea what could happen? It worked perfectly until yesterday, absolutely not other change but upgrade routerOS version to v6.45.5.

What software version were you using before? Does a reboot fix your issue?

Did you verify network connectivity to the RADIUS server?
It could be the result of the change in GRE tunnel firewall, so your RADIUS server may be unreachable because a GRE tunnel you use to reach it is now not working anymore.