I'm not saying the lock isn't real. I've accidentally exported to an open PDF hundreds of times. This specific issue is with DDP and has nothing to do with locks, at least in my case.
Here is the normal DDP Export Progress Bar -
Notice the "3 of 5 Pages"
Here is what happens on the second attempt -
No progress is made, the "1 of 5 pages" will never show up, and it will hang there, for as long as you want to watch it spin.
This is the error that happens if you export DDPs to an open file (locked) -
The DDP export will proceed as normal until the very end, when it shows this error. You'll never get to this on the second attempt, because it actually needs to export all the pages first, which it won't do.
Also, in reply to Kelvin. In my experience with this issue, it doesn't matter which option I choose (all pages, current page, or range), or how many pages are being exported (I've had this issue with 3 total pages, 8.5x11). Whatever you do on that second attempt, it will hang.