Fatal Error opening .map

I've been working on a map in VHE 3.4 (due to lower end laptop) for eventual import into Hammer 4 (end prodcut: HL2DM).

As part of ongoing work, I attempted to import the map into VHE4, but no matter how I try, it get this popup:
Title: Fatal Error
BlockArray < 0, 0 > - invalid block index
clicking OK closes Hammer.

The .map file is nothing special that I can tell. It's about 2400 lines / 178 KB. I've tried editing out the few light entities that were in the .map but it doesn't change the error.

VHE 3.4 opens the .map file fine.

The funny thing is the corresponding .rmf file opens and compiles just fine in Hammer 4.

While not a show stopper (at least not at this point), I'm curious as to what may be causing this.
Any ideas?

Thanks,
-Ryandor

If the RMF file works, surely you can just resave and then export that
to map? Forgive my rustiness with editing, but if the RMF works why
would you want to still use the incompatible .map ?

It really doesn't matter in the end, as long as I can get my VHE 3.4 map into VHE 4 so I can finish it into a HL2DM map. I was just curious as to possible erasons why the .map wouldn't import.

-Ryandor