communication after job offer and acceptance
It's time to get the phone! 

If you know some people working in your future institute, try to catch them by Skype. You can also look for some assistant or secretary telephone number in the institute. As long as you don't call by midnight, nobody will be annoyed by your call.

Yes, in Europe, things usually slow down in December / January, and mails accumulate or get trapped in spam filters. So, when in doubt, try and make a phone call.