I had the same issue. stopped working about a week ago. to get it working i had to do the following
1. log onto pia and generate a noe openvpn config https://www.privateinternetaccess.com/pages/ovpn-config-generator
2. stop the docker and copy the downloaded config over the top of the openvpn.ovpn file in appdata\activ-transmissionvpn\openvpn (or where ever you store it)
3. set the vpn_user and VPN_pass to my PIA login details, not the PPTP/L2TP/SOCKS Username and Password
4. change VPN type to custom
5. change the VPN_Remote to <server>.privacy.network
I'm not sure if all those steps are necessary but it is working for me now