I have found out there is problem in _parseRequest function (Parseing.cpp). In client buffer the data remains from previous cycle of _handleRequest function (8266WebServer.cpp). It generates false requests.
I have added client.stop() in _handleRequest as follows:
...
_currentClient.stop();
_currentClient = EthernetClient();
_currentUri = String();
}
After that problem disappeared.
Could someone investigate it if it is bug and can make troubles even in esp8266 projects?