Are you using some script or only RMXP in its default configuration?
In the latter case, this would be really strange.
In the former, just find every istance of this line:
and replace it with:
This will surely work, but is only a temporary medicine.
There should be something more complicated behind that error...
Are you using some difficulty change or enemy levels script?
These could be the root of all evil...
I hope this could help,