what exactly is the error, and where in the process does it occur (starting the launcher, logging in and going into the title screen, moving from title screen to character select, or from character select into the game)?

There have been some Win 8.1 specific issues that others may have overcome and may be able to help you if they've experienced exactly what you have. It could be something as simple as security blocking the new versions of the executables, or something much more complex. But in order for the players to help you, they will need more details.

Oh, and btw... you don't have SE's ear in these forums, as this is user to user support. To get assistance directly from SE, you need to use their support portal.