When Q++Studio is launched, it generates the list of Muslim years and dates. As a self-consistency check it verifies that the start of 1420 that it has just generated is April 17, 1999.
Usually this error can only happen after an update to Q++Studio where additional Muslim years have been added, and an error was made in entering this new data.
If you get this message, contact technical support, and in the meanwhile re-install a previous update of Q++Studio. You should not keep on using the current version of Q++Studio since all Muslim dates will be wrong.