I just updated my iPhone to iOS 14.4 last night and now Marvin 3 won't open.

I peeked at my old original Marvin and Gerty and both open just fine. I just happen to prefer Marvin 3 as it does what I need.
Does anyone have any suggestions on getting it to open or am I dead in the water?
Are there any other similar ebook readers that anyone can suggest or can you point me in the direction of where that discussion is.
Thank you!
Shykat