I created an AppImage that contains the BT #558 bug fix (0x00 vs. 0xff) AND with these 2 Experimental patches in to fix "nested edl undo crash" so you could test if you find the time.  The fix for 558 is the most important though and you already built that.  It is at:

   https://cinelerra-gg.org/download/testing/test_ubuntu16.AppImage

Locking and unlocking are extremely sensitive and can cause all kinds of weird crashes so I guess that is why the patch is called EXPERIMENTAL.  But could definitely be the missing part and fix the problem.