First, I apologize for not responding sooner. I completely missed the post.

The humidity(50-60%) isn't too high in my opinion. Are you using any kind of supplemental heat? Can you enlarge the top opening but still keep it secure? In my opinion natural humidity is much different then what we provide. In the wild the snakes can move and thus adjust. In an enclosure they are confined to a space. I don't think your snake is any more immune then any other snake(within reason, many variables)
Here's a link to the forum care sheet.

Garter Snake Forum - Garter Caresheet