When Mastering (on DP-008EX) after a mixdown, sometimes the process stops without a message of any kind. Retry gives the same result. Can this be because the mixdown level was too high?
This issue is due almost always to user error (e.g., setting of in/out points); or outdated firmware; and rarely an SD card issue; and almost never a software bug.
Since the problem only occurs "sometimes", not seeing the specific steps of your process listed, and not knowing whether followed consistently and correctly in every instance, makes a reasonable diagnosis difficult if not impossible. There are too many unknown variables.
Here's one example:
A quick forum search on "master stops" found this thread/discussion (among several).
Unfortunately, both members who posted the problem never bothered to report back nor in fact have been seen since the initial posts, so the outcome remains a mystery.