While working at a customer site, I was notified that the AAD Connect hasn't synchronized in the last 24 hours. Checking out the portal confirmed that even though password sync was working.
The error message didn't point to the actual problem. The event viewer on the Azure AD Connect server showed the error "Maintenance or sync cycle not being run because mutex acquisition failed. Will retry in 10 seconds". The event id was 904.
The issue turned out to be really simple. There was another admin session on the server with the Azure AD Connect configuration and MIIS client open. He/she may have been checking out a configuration, but failed to close all the tools.
I closed off the AAD Connect tools and fired a delta sync manually. Sure enough, the sync started working. Lesson learned - don't be confused with the error you get, always check the basics ;-)