Self-consistency error; value generated for start of 5759 is not 21-sep-98
When Q++ is launched, it generates the list of Jewish leap years and Jewish years' start dates. As a self-consistency check it verifies that the start of 5759 that it has just generated is September 21, 1998.
Usually this error can only happen after an update to Q++ where additional Jewish 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 Jewish dates will be wrong.