Been playing Rift on my iMac for a few months without problems.
Today they launched the Streaming Client and suddenly I couldn't play anymore.
Everytime i tried to hit "Update" in the patcher, the Rift Error window showed up.
What worked for me was the following:
Start the Rift Launcher
Login
Start Task Manager from your bottle
Kill "riftpatcherrorhandler.exe"
Click Update in the Rift Launcher
Wait for the files to convert
*Play!
This worked for me, hope it can help others.