Weird sign in issue

I can no longer sign into the AppStore and download / update apps.

I've read the trouble shoot web page of apple, without any success of solving this issue.

Once I write my Apple ID and password, the iPad looks like it's "thinking" for a second and then it shows me the sign in button again like I did not make a try a second before.

I have tried these methods for fixing this issue:

1. Trying a different wifi network.

2. Resetting time to automatically.

3. Resetting device's network settings.

4. Tried resigning I with my iPhone on the same wifi network (succeeded).

what at can I do?