Same Problem solved on my Phone.
This is not only a onedrive problem, i can't open any .kdbx file on my phone.
Before on my Nexus i have never such a problem, but on my new LG G Flex i can't open the file by selecting with the explorer, also i can't open from keepass Browse dialog.
I See the file (without extension) but it is greyed out (the same with all other files).
to solve this problem i had to reassign the .kdbx format with Keepaas and after this, i can open the files from OneDrive-App, Explorer and KeePaas Browse File Dialog
Greetings Thomas