iPad Lost access to DropBox on iPad2

I use an oldish second hand iPad 2, which I mainly use for Facebook and for checking the ebooks I design.

I am testing the latest ePub ebook I've designed, and have loaded it to Dropbox via my Windows 10 computer. I know it's there because I can open it and read it, and because I've downloaded it from Dropbox to my Levovo tablet with Calibre Companion.

But when I try to to download it into Bluefire Reader I get an error message 'The operation couldn't be completed. (NSURLErrorDomain error -1003.) I've downloaded many ebooks into Bluefire in the past using this method.

And when I open the Dropbox app on the iPad to download the file into iBooks although Dropbox shows the several other ebooks I've downloaded in this way it doesn't show the one I uploaded this afternoon. When I try to download an old file again I get the message 'Couldn't Load File.' When I select the icon above with the page and up pointing arrow I get the message 'Export Failed Dropbox was unable to download the file.'

What is going on? How do I fix this problem? Is there any other way of loading ebooks into iBooks and Bluefire from my computer without going through Dropbox?

Do I need to delete the Dropbox app and reinstall it? If so, how?