I bought a license for Indie, upgraded to .724 and started working with the Houdini Engine to Unreal at roughly the same time, so I can't be certain which caused the problem, but my middle mouse button no longer works in Houdini. It's not the mouse itself, because it works in every other program I use. If there's a setting to map the middle mouse button, I don't know where it is.

Update: Seems like it was just a problem with .742. I restarted and it still wasn't working so I checked Launcher for an update, and after updating to .751 I've got my middle mouse button back.

Wow, so I got my middle mouse button back, but I was unable to zoom in the viewport, either with the scroll wheel or the right mouse button. I switched to orthographic, and could zoom there, but not in perspective. What finally solved my problem was switching between first-person and tumbling modes, so if this isn't a curse that's particular to my computer, hope this helps.