Ive already replied in your editor topic, we should keep it in one place, in case anyone runs into same problem. I already posted, what I found there. I honestly dont know, WHAT causes the problem.
But I did get the same error today, while making a new ship. (aka no old X new version of the editor conflict would have anything to do with it) -
My work flow is allways the same.
I set the Center of the ship
Set the shields.
Bound polygons.
Add engines (edit engines right, after their placed)
Add weapons slots (first add all of them, then point, then set the weapon arcs.
Preview - Ship details, set the ID, name etc.
Done.
Exactly like this I make every ship and today I got a the same error. So I redid everything (deleted ship files, started over). Once I made the variants I saved it over and old variant(from the first attempt), which had the same name (forgot to delete it - just this one). So I over-written it. And.. then got another error, about the variant. SO I checked and as I stated in your topic- it had the OP 155/155, but had only two weapons on it. So I deleted the variant and made a new one - that worked.
So conclusion - I cannot reproduce the "glitch" that causes this behaviour. But its either a bad corruption from the ship file (last time making new and new variants from the "corrupted" .ship file gave me the same resulting error). Or it can (also) be problem while saving/creating the variant.
But its very strange, that Im the only one reporting this.. (could be my bad - but as I say, i use the above method over and over..)
D.