Loose cassette on road bike
Seems like I'm a Necromancer again...

Most likely a spacer. No, I take that back; Almost certainly a spacer. With the Hybrid cassette fitting correctly, it's quite often that they are slightly thicker, even by a couple of millimeters. An easy way to check is to check the limiter screws on the deraileur if you haven't adjusted them. If there's too far away from the wheel for where the jockey wheels line up, it's probably missing a spacer. If not, it needs a thicker spacer or more, assuming the gears were properly set up before you took it to pieces.

Also, good call, taking the cassette off to clean it. Too many times, people leave it on their wheel using a degreaser to clean it, and it breaks down the grease inside the hub and freehub body! Annoying because I usually get asked to replace them when it happens, then I lecture them about all the problems from doing so..... Boring!