OpenVPN server and PPTP clent simultanious usage leads to pfSense hang

[knst]

I need help with following problem:

My ISP provides internet access by means of PPTP (WAN is connected to ISP network, PPTP client connects to the server in this network and internet becomes available)
If I try to enable OpenVPN server, changes in firewall rules, NAT or static routes lead to pfSense hang and there hardware reset is only way to bring it up.

Thanks in advance