When Q++ 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++ 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++. You should not keep on using the current version of Q++ since all Muslim dates will be wrong.