This can happen if the server’s or your connection times out for quite a while or breaks up completely. A common solution is to update your modem/router’s firmware. Also, make sure that you aren’t running “Cucusoft Net Guard” which is incompatible with BE. Other than that, please ensure that there are no problems with your internet connection (you might want to contact your ISP).
This answer is copied from BE's FAQ and slightly adapted to be Unturned-specific. Click here to visit the BE support page.