Peter,
Thanks for posting to the Forums! I have had experience with a Jura Impressa and Jura Z6, so maybe I can help a bit.
As for the water tanks on some models there is a magnetic float that goes up and down slightly with water level. It’s very obvious, it’s black in color and near the bottom. With the model that has a float (Sources online show you have a float inside your water tank), mold and mildew can grow in this area that holds the float and it can become stuck and make the machine act very finicky. So use some mild soap and water to clean it out, use a long brush to agitate the box that contains the float to attempt to break up whatever could be keeping it from moving freely. Make sure to rinse with hot water thoroughly to remove all soap residues. I recommend using Dawn.
If you do have a float but it is moving freely the sensor inside the machine which detects the magnet is going bad, this’ll require some attention from a Jura Technician or someone in your area who works on coffee machines in general
Happy Fixing!
Noah