Is Autor masculine or neuter? Duolingo says it's "use 'dem' for neuter nouns like 'Autor'" but Wiktionary says Autor is a masculine noun. Canoo.net also says Autor is a masculine noun.
It's a masculine noun. Das is used for neutral, and Der for masculine. However in this example, it is in the dative case, so both neuter and masculine articles change to dem.
It is masculine. Duolingo's dictionary is really poor. Don't rely on it.
Anyone else noticed that "ist" is also accepted?
If it is, it shouldn't. Although 'isst' and 'ist' are homophones, "Der Herausgeber ist mit dem Autor" is ungrammatical in German.
Because it should be "bei dem Autor"?
Yes, or 'zusammen/gemeinsam mit dem Autor'.