We recommend to use Citrix Receiver Version 4.5 which has been released on September, 14th 2016. This version fixes issues with freezing sessions and was stable in previous tests.
On windows platform, Internet Explorer is recommended.
You can use the following browsers
Chrome is currently not supported
Add UNISERV logon page to trusted sites list:
Internet Options -> Security-> Trusted Sites
Make sure, that your Browser accepts Cookies from “https://ud.uniserv-online.de“