First of all, it's not an inherent quality of the paper unless it's age fogged or has been subjected to truly unusual storage conditions. Adox MCC hasn't been around long enough for it to be age fogged. Prolonged exposure to elevated temperatures or exposure to fumes from sulfide toners can fog paper; so if neither of those conditions exist, there's no reason the print should be fogged even with an exposure that long. I routinely shoot for exposures of a minute, or sometimes even more if I have some complicated dodging and burning maneuvers to make, and don't have that problem. For an exposure of over three minutes, I'd be checking everything for stray reflections and light leaks, and addressing them as best I could. Chances are better than good that that's whats going on with your setup. Check your enlarger for light leaks and cover them up. Make the walls near the enlarger as non-reflective as possible. Line them with something matte black if you can't paint it matte black. Do the same with the ceiling. That should fix it.