Why isn't Exchange activesync working over WIFI networks?

[AC Question]

Exchange activesync not working over WIFI networks

I'm running S4 and since i did the upgrade to android 5, i cannot sync anymore over WIFI networks. Using 3G/4G networks, it is working good. The error message I got over WIFI is 'unable to connect to the server'. I tried several WIFI networks but always the same issue. I tried also to remove the email account configuration on the phone and re-do it but same result. Any idea/tips to sort out this issue ?

Thanks