This error is due to a communication error in the printer. Since there can be several causes for this, we recommend to first reboot the Ultimaker S5 / Ultimaker S3 by using the on/off button.
Please contact your local service provider for help and send them the log files of your printer. If a USB stick was inserted into the printer when the error appeared, the log files were automatically stored on the USB stick.
Otherwise, please insert a USB stick, then in the menu navigate to Preferences > Maintenance > Save logs files to USB. Send all log files to your support contact, so they can view them and provide further assistance.
If the problem still persists, please contact support by raising a support request above.