I encountered an issue where my Ubuntu 17.04 VM would enter maintenance mode on every boot. However, if I pressed Ctrl-D, or exited the maintenance shell and continued the boot, the system would start up fine, with no failed jobs.
I eventually narrowed it down to a my 9pfs Virtual filesystem mount. The job was hanging for no discernible reason. Running it manually from the maintenance mode would succeed. Enabling debugging for SystemD didn't produce any more helpful errors.