Strange error message when I try to forward an email

Hi everyone, I returned from an errand and tried to forward an email receipt to my operations manager, but every time I try to forward or open a draft I get this error message:

When I reload the page, it looks normal, but if I try to edit the draft of the forwarded message I get this again. My internet connection appears otherwise stable and I'm not having any difficulty in opening other web pages. Has anyone seen this before? What's the cause and resolution?

Thank you!