If you get the following error while connecting to Subversion using TortoiseSVN:

Error: PROPFIND request failed on '/path/to/SVN' 
Error: PROPFIND '/path/to/SVN': could not connect to server (https://sample.svnserver.domain) 

and the Subversion server is using a server certificate from a non-trusted authority (i.e., they created it themselves), TortoiseSVN will fail with this error message but without telling you why.

Solution: 

First make sure it's not a proxy issue. Then go into the repo-browser and navigate to the Subversion server URL. This time you'll get prompted to accept the server certificate (permanently) and to enter your username and password. If you choose Accept Permanently, you'll never need to do this again.