All you need is to pinpoint the leak and then with a few simple tools you should be able to do it yourself.
Is it leaking inside the machine case? Like water is running out the bottom of the case? I had a problem like that once, I forget what it was but I think it was just one of the fittings in the solution system that needed taking apart, old PTFE removed and then re-tightening up with fresh PTFE tape on the thread.
You really need to run the pump with the machine open..... but this is the problem with a ninja, because it hinges to get inside - you have to be a bit careful because the tank is now almost horizontal! But it can be done, trust me
Once the pump is running it should be fairly easy to spot the leak. I'd have a few towels handy and dry the inside so you can see where it gets wet. Might be a leaking fitting as I said, or could eb a split hose or where the hose goes on to the pump inlet/outlet, they are a barbed spigot, sometimes that needs re-fixing.
Could also be getting in where the heater, sensor or pump inlet go through to the bottom of the tank but very unlikely. Have a look and see what you find, yell if you need any more help
