Hi!
Since a few months it is necessary to establish a vpn connection if you e.g. want to connect to your TU webspace.
So you may try to connect to the TU via vpn and then try to establish the connection via putty.
Downloading and installing WinSCP following the instructions was really easy.
Without any further complications I could connect to two different server machines.
Maybe simply the putty attempt was made without secure socket,
while with WinSCP this was the bvious option.
But I don't really remember the whole story weeks ago under time pressure.