Internal server error on ajax post

I have a very unusual issue when submitting contact form from my website. It worked ok until a few days ago and I did not make any editing to the template. However, each time I try to submit the form I get a server internal error. I contacted my host company, they checked the log files and said it must be an error related to EE. What’s really odd is if I submit the form from a computer which is outside my home network, it is ok. Tried on my iphone, when it’s wifi connected I get the server error, if its not wifi connected then it works. Tried everything I could think of.. cleaning cache and cookies of my browsers, checked htaccess files and index.php permissions. Any ideas?