What happened?
This is a generic error message indicating that a software fault was detected on the MMU unit. This might be an error that doesn’t have its own specific error code yet.
How to fix it?
Make sure that are using the correct up-to-date firmware for both the printer and the MMU unit: Firmware download.
If that does not solve the error, take note of all the actions that preceded the error happening. Try to reproduce the error, and save the log.
After that, contact our カスタマーサポート, or report the issue directly in the Prusa GitHub page.