my challenge to you is :-
provide some fully compliable code that demonstrates the problem , I would bet you cannot
To do that I should send to you all the project! Which is not possible!

But if you read better my first post, you will realize that I do see the CRC value of 253 on the client side, which is an android tablet. The CRC is sent by the pic via esp8266 (as you can easily observe from my code in post 1) and received by the android tablet (acting as a client via an apk written in basic4android by myself). Now the state of the art is: IF I DO NOT INVERT THE BYTE VALUE THE CRC RECEIVED IS CORRECT IF I DO INVERT INVERT THE BYTE VALUE THE CRC RECEIVED IS ALWAYS 253!

At this stage if we exclude the pic then the problem could be the esp or my apk! Need time to investigate in the meantime I will not invert the byte value (action that doesn't add much to the CRC) since everything works very well at the moment.

Alberto