While there may be numerous causes for a reel-lock, we encountered a surprising one:
The machine would often fail while pre-rolling for an edit, after it had been working quite well for an editing session, and would need switching off and back on again, every now and then, to reset it. Much later, the fault worsened until there was next to no chance at getting it to work again, once the error occurred, until it had completely cooled down.
Poking around the motor drive board under the chassis, the servo ICs were very hot. Way too hot for tiny heatsinks that had no fan blowing over them, in my opinion, and some redesigning is in order (perhaps another fan, or diverting some of the airflow of the existing fan). It appeared that one IC had dry joints that were being shown up by the overheating, and resoldering the pins around it resolved the problem. But the fix proved to be temporary.
Eventually, I discovered that the capstan needed lubricating. While it moved freely, it didn't move freely enough, and the extra drag meant the motors had to work just that little bit harder to drive the tape. Just a tiny drop of oil is needed, and be careful that you do this in a way that no oil will ever get into the tape path, as you will never be able to totally remove oil from whatever it touches.
You can do what watch repairers would do: Don't try to squeeze a tiny amount of oil into where it needs to go. Dip a toothpick into your oil, and carry a drop on its tip over to where you're going to oil. That's about all the amount of oil you're going to need.
Be very careful when cleaning the tape path. If cleaning fluid runs down the capstan shaft into its bearing, it flushes out the lubrication. Likewise with any other moving parts. Merely moisten your cleaning tool, don't have it dripping wet.