Maybe set it to an incorrect path and check if you get the same error again. Also, I’m not sure how curl on Windows works. Does php install it (as a curl.dll, curl.exe or so) or do you have to install it separately? If you can use curl from the command line on Windows you can check if it shows the same problem as via php.
I had another issue with curl on Linux with this update and first thought, the way above might be the solution. So, I tried it and at one point I mistyped the path and I think I got exactly that error above. So, it very much looks to me like curl isn’t able to find this path. Either because it is not allowed or because it needs it differently (maybe slashes instead of backslashes or so?).
Why it becomes an issue only now? Because 2.12.0 had new update code that wants to retrieve the files via https.