TortoisePlink uses the standard plink code, but is compiled as a Windowless app, so there's nowhere to send the error messages to. In TSVN settings->network, set the SSH client box to use standard plink, where the error messages are displayed in a command box. Once that is working, use TortoisePlink with the same parameters.
"could not write to standard output" means that Plink wanted to throw an error, but since TortoisePlink doesn't provide a DOS-box there's no standard output to write the error message to.
So something with the setup is wrong. Try with the normal plink program and see there what error message really is thrown and then fix the setup.
If normal plink just hangs, then the wrong params are passed to it (settings dialog, network tab).
Another possibility is that the SSH daemon is most likey not able to find the svnserve binary. Login with your target user (here myuser) into the server and type "which svnserve". If you dont see the path to the binary, make this file (and most likely the other subversion binaries) globally accessible to this user.