Hi, @virgil. (Please allow me: KeepassDX or KeepassXC? AFAIK, the former is only for Android.) Anyway, I have used them all, so maybe I can help.
If you are a hundred percent sure that there was no mistake in entering the key, my suggestion would be to:
save the KeepassDX database in a .kbd file* (keepass 1) using a simple password key;
move the file to your SFOS phone;
then to edit it with ownkeepass there (change password key, save as new .kdbx file).
*or, save it in all the different formats, to cover all the bases.
As for what happened with the first import, I am sorry I do not have any ideas.