I have the same problem with water on top of the cassette. I got fed up today so removed the toilet to sort it out. It turned out to be a tiny leak from the joint from the hose to the electric valve. The rubber washer was full of limescale and the screw tightener was just a little slack. I changed the rubber washer (same as in normal garden hose fittings) and also used ptfe tape on the thread. After a good cleaning inside, refitted the toilet and made sure it wasn't leaking.
To make it easier, the toilet bowl can be removed by undoing 10 screws from where the cassette fits. (5 plastic fittings to let the bowl turn)
Easy to relocate and refit.
only bad bit was moving all the things she had stored under the bed so I could get in....