How do I get DDO to run under windows 8.1?

Error 201 means that the game cannot access the game's files. It can occur when the permissions are set to administrator only or if a previous patch failed.

Best is to reinstall the game fresh from the interwebs. I would also advise to install it into userspace instead of global space, which can also be done by simply installing it to a different partition rather than the usual C:\Program Files.

This doesn't answer the root problem, but I found that the Play Instantly button and it's Happy Cloud version would install and run when the other installer would seem to do a full install but the game installation couldn't reach the game servers on a new Windows 8.1 system.

It's possible that I had a corrupted download of the full install file, but I downloaded it and ran it and it gave no error messages. I would hope that an online game vendor's installer would do a check sum at the very least on itself.