thanks for your answer!
As I didn’t want to set up another ESP, I tracked down the problem and I think, I finally found a solution 
the pilight port is random since pilight version 7.0 if not defined in the config.json (which is not the case with the default config!).
So I thought the binding uses the webserver and pointed it to port 5001. Therefore the exception was thrown (unexpected character).
I defined the port 5000 in the pilight config.json and changed it in the pilight.cfg (back from 5001 to 5000) in the services directory.
pilight.log
[Jan 30 13:42:07:650354] pilight-daemon: INFO: new Client, ip: 127.0.0.1, port: 42942
openhab.log
2018-01-30 14:42:07.034 [INFO ] [ternal.PilightGenericBindingProvider] - pilight:rpi item Wohnzimmer_Stehlampe_pilight bound to device Stehlampe
2018-01-30 14:42:07.993 [INFO ] [ng.pilight.internal.PilightConnector] - Established connection to pilight server at localhost:5000
success!! (this is with the binding version 1.9.0 btw, didn’t try 1.12.0 anymore)
it should work now, will try immediately, when I get home 
Edit: confirmed working, items are updated, when signal from remote is received