what if i use mockhttpservletrequest?

Question

You are right, this exception is thrown because the stubbing setup does not match the objects that were passed during test execution.

The .toString() represents this inside the Stacktrace:

[email protected]__, "603b408b-4eb2-4d01-ab90-d30277874415"            
[email protected]__, "603b408b-4eb2-4d01-ab90-d30277874415"

The id argument does match, but not the instance of your MockHttpServletRequest.

You can solve this by a more open stub setup, that does not check on equality of the MockHttpServletRequest

when(this.service.validateDeletingProcessBoard(any(MockHttpServletRequest.class),eq(id))).thenReturn(object);

Make sure to use org.mockito.ArgumentMatchers for both any() and eq().