Sadly, when iOS 9 was introduced (I think that was the version...), Apple made some changes in how apps could interact. I'm not a tech guy that understands the details, but essentially, those changes "broke" the connection between Calibre and Marvin. Apparently, there is a way that Marvin could be fixed, but the developer of Marvin has been (so it seems) focused on other projects. So, for the time being, the best way to get books into Marvin is via the OPDS server instead of directly connecting the iOS device to the computer.
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