Found the reason: Compression is not enabled in new versions of the OpenVPN Android app (considered insecure). Solution was removing the compression-option from the server and the client config file (“compress lzo”).
interesting, thanks for info. Here is full list of derp.options: https://community.openvpn.net/openvpn/wiki/DeprecatedOptions
… “comp-lzo” should still work (with 2.4.x and older versions), if you have newer client, it will require new format/syntax 
(and not sure if the server can have deprecated or new one…)
thanks @drhirn - this fixed indeed the problem!. and it took only few guys. 
i like the forum is active. ten points!