when the scoreDefinitionType does not match/is not compatible to the solutionScore class the exception says it is an illegal argument because the classes are assignable.

There seems to be a "not" missing in the text.

example exception output from my test:

java.lang.IllegalArgumentException: The solutionScoreClass (class org.optaplanner.core.api.score.buildin.hardmediumsoft.HardMediumSoftScore) of solutionClass (class xxx.ResourcePlanningSolution) is the same or a superclass as the scoreDefinition's scoreClass (class org.optaplanner.core.api.score.buildin.hardsoft.HardSoftScore).