cant login to facebook in android applications - FB dialog is opened but keep "loading"
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I have mobile app (for android) with Facebook login (FB SDK).
It worked fine , and it stopped working - the dialog for login is opened but it keep "loading".
I tried it on another app - same result , i tried in another device - same result.
both devices were tried in the same wifi.
when i tried the app , with 3g (not wifi) on the device it worked (able to login).
I want to work on the wifi.
why? and how do i solve it?

more info:
1. the error i get is : 
Login failed: com.facebook.android.DialogError: The server failed to communicate. Try again later.
2. I tried it both in phonegap app and native app (android) - same result.
3. it worked fine till yesterday.
4. i reset the devices several times
5. i uninstall/install the apps