The door was open by the employee.
The door was opened by the employee.
Can we use "by somebody" after "open" No (it's an ajective, not a verb). Are both sentences grammaticalli correct? Why?
In the first sentence, as you've noticed, open is an adjective, and therefore cannot be followed by a by-phrase.
In the second sentence, opened is part of the true passive construction, and a by-phrase can be applied easily.