If you are encountering this error, the Ultimaker 2+ Connect has detected an invalid reading with your heated bed. Most likely a connection is loose.
To troubleshoot this error, first visually inspect the cables behind the heated bed assembly. They should be firmly attached to the build plate and not frayed.
If the cables are firmly attached, you can look at the connection between the cables and the motherboard. Open up the bottom cover by removing the two screws holding it in place, and slide the cover out.
Look at all the connections to the motherboard. If there are any loose, or frayed it may be that you need to replace the connector to the heated bed.
If the problem is persisting, raise a support ticket above.