We are behind a corporate firewall and therefore cannot make direct SSH connections to our Stash server.
So we need to use a Proxy for SSH connections. With PuTTY/Plink this works by creating a profile for the host with the Proxy settings.

https://monkeyswithbuttons.wordpress.com/2010/10/01/ssh-proxycommand-and-putty/
https://blog.terminal.com/setting-up-putty-with-terminals-ssh-proxy/

After settings PuTTY up like described in the articles above I am able to git clone repositories from the command line via SSH through our proxy.
Anyways, trying to clone the same repositories in SourceTree will result in an network connection error.

Couldn't figure out why it does not work with SourceTree.