Can't update newsletter subscriptions

I’m currently getting (at least) the dev newsletter and firefox news regularly and I can’t turn them off. When I follow the steps to update my newsletter subscriptions (where lots of other subjects are already checked), the changes won’t stick. Previously I was getting a super-unhelpful, very general red “error occurred” message, but now the whole page just whites out on clicking “Save Preferences”:

This has been going on for months, and it’s driving me nuts. I don’t know who handles newsletters as opposed to mailing lists (which is a different system), so any help as to where or whom I should report this bug would be great.