by Gary Scott.
Problem solved! Here's what hosting support wrote:
After investigation I have found that Mod_security from server was causing this issues.
I have disabled it for your account and was able to open "Grade" on my end.
Not sure how I feel about having mod_security disabled, but upon further research, it doesn't seem like it's that big of a deal.
Thanks to everyone who beat their head against the wall with me on this one.