Let's see, you had a software problem, that required the owners attention. And while on the phone, you discussed, what the GM, didn't have any answers to? Then, the owner was miffed(so to speak), that you didn't bring up other issues, the GM, clearly stated to you, that she was going to take care of?
And someone, left an opened box, without a inventory sheet/receipt of contents, for you to put away? Now, who asked you to put away the box? And while on vacation, which employee was negligent in the handling of inventory?

And you are nervous, because the two bosses, seem to be in a heated moment and you returned from two weeks off? Yeah, easier said than done, but realizing they are both playing the 'blame game', is a mentality to keep in mind when dealing with confrontation from bosses in the work place.

Hope all goes well, with your presentation...