I have been using my phone for a month now. I have an open exchange account on Strato.de and it worked fine until now.
Today my contacts are empty, the phone shows only those storend on SIM.
Also my calendar appears empty.
Just emails are still synchronized.
I did not change any settings, all three are marked for sync.
Other clients (Web browser) show that the account is OK.
If you did not apply any updates or install any apps before this occured, I think it’s got something to do with the server. I suggest you to contact your provider and ask if they changed something in their setup.
Possibly some other app (maybe Google?) messed up the account settings of your Exchange account.
Deleting the account and installing it again should be the first thing to try. Good you came up with it yourself!