EDIT:
I have tried 32 and 64 bits accdr and accde files. They work great on my Office 2016 installation but I still get the "unrecognized format" error.
Any more clues? I think I saw that a special switch was needed to start Access 365 in runtime mode. Does that ring a bell to anyone?
I have an accdr file that is used by a bunch of clients (non profit orgs). One of them has Office 365 Business edition and it just won't open the accdr file, giving the "Unrecognized Database Format" error.
Can that be fixed through a switch in the command line? Do I need to create the accdb in O365 (it was created in 2016)? Can O365 be 32 bits? The accdr was done on an x64 office installation.
Make sure that the file is not corrupted and that it's working on another machine. If the file is OK, then download and install Microsoft Access 2016 Runtime, and the file will work.
Download the right edition for your OS. 32-bit or 64-bit.
Note: If the file was created with Access 64-bit it cannot work on 32-bit OS.