I've got a Mikrotik CCR1036 with routeros version 6.49.7 working as VPN server. Now I'm trying to authenticate users against a radius server (a windows domain controller).

I configured the mikrotik side like this: Mikrotik logs shows the message "pptp,ppp,error <1244> user xxx authentication failed - radius timeout", like it was trying to connect to the server but the counters on status tab shows 0 requests and sniffing the traffic with wireshark at server it shows no radius packets incoming. I also used the packet sniffer tool at mikrotik and I see no radius traffic outgoing (pings from mikrotik to radius server works ok and even tried completely disabling windows firewall).

I think the radius server is working OK because I checked it with radtest from linux and it sends queries ok.

I'm not sure if it is still a thing, but years ago when you first setup a RADIUS client for VPN or PPP you then had to reboot the MT router before it would begin to send the requests. Have not tested that particular requirement in several years, but I believe at least as recently as 6.45 it was still a thing, if you haven't rebooted give it a try.

also good to double check your input and output chain firewall rules, easy to get messed up there.