Problem solved: there were more issues.
1st: my router's security software (AiProtection) blocked opera VPN (I don't know why, previously it hasn't caused any problem)
2nd: when I've tried to connect my machines directly to ISP's cable, without the router, the interface ppp0's mtu was set to 1500 by default and it caused some errors. After reducing mtu to 1400 , communication issues was eliminated.

etc.

@victorsugo
a small addition: I'm using a DSL like service with dynamic IP. When I restart pppd on thre router, to get a new public IP, then I can connect to Opera VPN, if I start VPN connection in some seconds after reconnecting to my ISP, and it keeps working until closing opera.
(Interesting: the error messages are shown even the connection to the VPN is successfull)

Some days before the VPN worked perfectly, but about two or three days ago it stopped working.
If I start opera from command line, I see these error messages:

[31900:32005:1116/092320.067371:ERROR:ssl_client_socket_impl.cc(1050)] handshake failed; returned -1, SSL error code 1, net_error -107
[31900:32005:1116/092320.204343:ERROR:ssl_client_socket_impl.cc(1050)] handshake failed; returned -1, SSL error code 1, net_error -107
[31900:32005:1116/092321.260326:ERROR:ssl_client_socket_impl.cc(1050)] handshake failed; returned -1, SSL error code 1, net_error -107
[31900:32005:1116/092325.082627:ERROR:ssl_client_socket_impl.cc(1050)] handshake failed; returned -1, SSL error code 1, net_error -107

I'm using Ubuntu 16.04, but I've tried it on an old machine which runs Ubuntu 14.04. The result was the same.
What can I do if I'd like to use Opera VPN?