I did not have network settings changed in any way and thus most of the stuff presented here did not apply to me. After messing around a lot the comment about the virus scanner got me on the right track: There are some virus scanners like McAfee, that protect certain areas of the system directories and make them read-only. When you connect to a server for the first time, Tortoise SVN tries to write the certificate on one of these files which fails due to the protection. Switch off the protection briefly, start the check out and after the certificate dialog, you can switch it back on. This at least worked for me.