I might have had a similar problem on one of my machines. But as that one also has that strange 0x40 error it might also have arisen from that error.
In any case: None of the pre-June BEs would boot properly - lots of red indications while trying to load modules etc.
Initially I just wanted to upgrade the latest BE via update. But as none of the BEs would boot I just installed from the USB image into a new BE and deleted all the other BEs - I didn't think I could repair them (didn't think of @mer's trick), and honestly I didn't really care.
For one I'm only still testing TrueOS and that system is not even my main test system, and secondly, the June 2017 image brings so much new stuff that I wouldn't need an older BE anyway. The things I use work (except for the mentioned minor bug) as far as I can tell.
So, I also had this strange problem that none of the existing BEs (I guess they were all based of the April 2017 image) even though they worked fine last time I fired up that machine.
But as I mentioned, between the BTX loader and the BSD (boot)loader I get that not very healthy sounding error message - so my problem might have a different cause.