Unfortunately, Promo, I think he's serious. I have not seen that error.
When you reload, try stopping after each step and seeing if the game runs (i.e. load Mech 4. Stop. See if it runs. If yes, then load PR1. Stop. See if it runs, continue) This can allow you to narrow down what part of your load is causing the problems.
Just a thought. I haven't reloaded Mercs in a while, so I am not really up on the possible errors.