Unable to download ePubs on new iPhone 12

[sglinert]

I've been using Marvin and KyBooks for years on my iPads with no trouble. I just got a new iPhone 12 Pro, installed both Marvin and Kybooks...and can't download most ePubs. I get a "not a valid EPUB book" message in both applications. A very few of my books do download with no problem. Worse, when I use iMazing and just drag the books to the documents folder for each app, both apps reject the books as invalid formats. These books work fine on the iPads and in iBooks.

I can find no way to actually download books at all in Marvin SxS--there's no Cloud or Home button. I've never used this program--I use the old verson with the red icon on my iPads. Is there a secret button somewhere for downloading?

The old version of Marvin works with the Calibre server just fine at first, pulls up the library list, and then says the book is an invalid format.

I disconnected the charging cable from the iPhone, and now both Marvins say either the book isn't a valid ePub, or the server is off-line. I copied the Calibre server address from my iPad and pasted it into the iPhone. Marvin can find the library and shows the list of options (By Title, etc), but I can't get books to actually download.

I am completely stumped and more than a little frustrated.