Why does TortoisePlink.exe show "No supported authentication methods available" where plink.exe works fine?
Solution 1:
There is a problem with 64-bit TortoiseSVN working with 32-bit pageant. Either use 32-bit TortoiseSVN (IMO there is no real need for it to be 64-bit). Unofficially, the devs only supply 32-bit binaries. A 64-bit build of PuTTY is available here.
It seems that the TortoiseSVN devs put the blame on PuTTY...