Tortoise SVN solution:
I just had this happen to me today running on Windows 7 trying to use svn from git bash. Any command I used that tried to hit the server would get the same "Server certificate verification failed: certificate has expired, issuer is not trusted". Usually, just continuing to try to hit the server will cause svn to then ask you if you trust the server. If you say yes, then all is well.
However, this time, none of that was working. I finally noticed that the svn I was using was part of the installation of Tortoise SVN. So I pulled up File Explorer and tried updating my directory through the Tortoise controls.
Tortoise handled everything perfectly. I got a window asking if I trusted the server. I said yes permanently. Then it asked me for my username and password. After I gave that, Tortoise updated the folder, and when I tried interacting with the server via the command line, that was just fine, too.
Bottom line: If you are on Windows using the command line svn as part of the Tortoise package, try updating directly through the Tortoise controls.