Right. The /rootsuffix option basically forces VS to run with a different registry root. In many ways, it's basically like wiping out the entire VS configuration set and starting from scratch in a sandbox session. As this resolves the problem temporarily, it seems that your VS registry data has been corrupted somehow.
It's impossible to know what has caused this corruption, but it's also likely to affect other features inside the IDE. There are two ways you can proceed from here:
1. Continue to run VS using /rootsuffix whenever you load it. This is a viable option if you just adjust the shortcut you use to launch it. As long as the IDE is launched with the same rootsuffix each time, the settings will ensure and you can use it as normal.
2. Reinstall VS from scratch. It's possible you may need to wipe out the VS registry if the uninstaller doesn't do this for you. It's hard for me to advise you on which keys can be safely removed or how to remove them, as this is more the area of the VS dev team.
Since this is just a CTP, I'd recommend the first option (/rootsuffix on a shortcut). Likely you'll be installing the full version of 2015 when its released later, and there's a chance this install step will resolve the problem.
