Hi Martin,
I know of such a bug that is caused by corrupt Custom Voices.
If you have installed some Custom Voices (.uvn or .uvd format), I recommend that you call up all Custom Voices one after the other. If the error I described occurs again, I recommend deleting the relevant Custom Voice(s) from the Tyros4 (in "Library Edit").
Note: This does NOT delete the Wave data of the Custom Voice(s) from the Expansion Memory Module. If such corrupt Custom Voices take up a lot of Wave storage space, you would have to delete the Wave data separately. In this case, please refer to the Tyros4 Reference Manual, starting on page 17 (Voice Creator).
Best regards,
Chris