I had a team site owner contact me wondering why they can't sync to a new file library that they've created. (See image.)
So I went and did some exploring. They added a bunch of content types, and changed the default content type to a visio file and changed the default template to a visio. I got the document content type to be default and added a word document to be the template. Didn't make any difference.
They have content approval turned on, I thought that might cause it. It doesn't.
They have check out turned on to edit, that didn't matter either.
I thought it might be an Asset Library, it isn't.
You can still connect to Windows File Explorer up on the Library ribbon. When I copy the library and create a template of it, it still retains the same behaviour.
When I create a new document library in that site, it works fine.
I must be missing something, but I'm running out of ideas. Any assistance would be appreciated.