Grammar errors or grammatical errors? Which is correct?

Grammatical errors is correct, because, if you wished to say it archaically, you would say "errors of grammar" or "errors in grammar". The first would require grammar to be put in the genitive case, which is Grammatical. The second, as there is no ablative case in English, is correct as is. "Grammar errors" has the word grammar in the nominative case, which, strictly speaking, should can not be used correctly.