SharePoint 2013 document generation -Can’t open document

I’m trying to use a document library to generate documents.

I have created 6 content types (as I need multiple templates) and associated document templates to them. I read somewhere I should be using .docx instead of .dotx as a template.

But I can’t get it to work. I get this error:

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We’re sorry. We can’t open DocumentName.docx because we found a problem with its contents.”