A couple other people ran into problems if their learned skill level and equipment bonuses for Magic Barrier added up to 5 or more.
Try using
iZakaroN's SaveEditor (or from
Kiya's site, with description) to unlearn a level of the skill, or switch to a different helmet. You could use also
DAD's item editor on a different helmet to recreate your current one, minus the skill bonus, if you are so inclined.
You can use the save editor repeatedly to raise your resistances. The resistance bonus when learning Magic Barrier is applied immediately, so if you remove all skill points from it, load the game, max the skill, save, edit and remove skill points, load and re-learn the skill, you can slowly replace the lost resistance points.
Resistances can also be hex edited; see
Some more hex offsets... (and for reference,
Teleporting Anywhere, via hex editor).