500 Internal Server Error i cant see any code to help :D

See if it fails on your computer using 3g. If it does have a close look at the requests being sent in your chrome inspector and try to log the requests being receive on your server and compare the two. Some 3g providers add some nasty caching to your requests which can mess with the headers and even content. I've dealt with a few ajax errors that were only found on Vodafone.

Thanks for you reply guys. Yeah, it fails as well from the computer....I compared both headers using 3G and wi-fi and the Status code is '200 OK' for both, but I've noticed a few differences:
Request Headers: In the 3G request it has 'Cache-Control:max-age=0' while in the wi-fi I don't have it;
Response Headers: 'Connection:' in 3G is 'close' while in wi-fi is 'Keep-Alive', and I think that because of that in the wi-fi I have the following data which I don't have in 3G: 'Keep-Alive:', 'Server:', 'Set-Cookie:', 'Transfer-Encoding:' and 'X-Powered-By:'.

Anything that might be related to the problem?