Im not sure why you get a 500 error... 500 errors are usually caused by the server and related to the server. for example my url is:
http://www.revolutionart.nl/wp-content/uploads/2015/08/m590_shotgun.mview?thumb=1
and the page itself http://www.revolutionart.nl/project/mossberg-590/
Not a single error regarding the thumb. Either its BPS or something is up with your Server. Since BPS is a security plugin it could be that BPS is trying to block it for some reason, even tho it still works (well the url the thumb itself isn't showing up) You may want to check BPS or ask it on their forums. Can't do much about that either tho since thats how Marmoset renders the thumbs into their viewer.
Let me know!