I have renamed the only Admin-account to 'beheerder@ourdomain.onmicrosoft.com' to exclude it from ADFS, prior to enabling dirsync. I could use that account to log on and used it to set up dirsync.
However, after the initial sync, that account no longer works. Dirsync stopped working, and I cannot log in using that account. When I attempt a password recovery, it says the user id does not exist.
As this was the only admin account, I can no longer log on to Office 365 with an admin account. Regular users (at least, those that were synced by the initial pass) can log on so ADFS works and dirsync did sync once.
As I see it, the only way out of this mess is by either undeleting the 'beheerder@ourdomain.onmicrosoft.com' account or by providing admin-rights to one of the existing users. I'd prefer the first as it's the safest option and have already disabled dirsync to prevent further accidental removal.
I would create a support ticket using regular channels, but as I no longer have an admin account to log on to, I'm hoping it can be resolved this way.
Note that I've substituted the real domain with 'ourdomain' for safety reasons. I'll PM you the real one if it's required.