If you are using Wunderlist 2 behind a corporate proxy (i.e. Microsoft ISA) which uses NTLM as authentication you might have troubles with login and synchronization.
But there is a fix for that. In order to get the authentication working we need to tweak the application configuration a bit. The tricky part is to find the application location because Wunderlist is deployed via ClickOnce and the exact path is kinda random. The easiest way to find the location of the executable is to run Wunderlist, open the TaskManager and choose "Open File Location" on wunderlist.exe.
If you managed that, open "wunderlist.exe.config" and add the following somewhere between
<defaultProxy enabled="true" useDefaultCredentials="true" />
This tells the .NET Proxyclass which autodetects the system configuration to use the credentials of the current user for authentication.
Hint: I forgot to mention that this works only with the Windows client of Wunderlist.