Hey, a similar issue happened to us today. We added a new customType and immediately the client functions stopped working because of the that error - Unknown type.
The solution was kind of unexpected - we needed to publish the first document of that type (even though we have the preview for drafts configured).
It seems like a bug to me. It should accept the valid customType value even if there are no documents of that type published. Otherwise any content editor can break the whole website if they remove all docs of any type (which is a rare scenario, but completely realistic and valid from the content editor perspective).