Red warning light permanently on Theford C 280?

Have 2011 Hymer Van 562 with Thetford C 280 Cassette Toilet. As the Van new only used a couple of times and after emptying toilet last trip and adding Blue fluid Red warning light came on and stays on even though toilet empty.!! Have removed Cassette to see what happens and Red light flashes continuously. Tried flushing some water down Cassette and both Red, 2 yellow and Fan lights come on for few minutes then back to just Red.
Can anyone help please.? Thanks