I think this may be a permission issue. Could you confirm with your Super Admins that you have "Publish" and "Write" permissions under the Marketing Tab > Marketing Tools > Emails in Users and team in the Settings? Steps here.
Strange error when trying to select an email template.
I would try logging out and logging back in again if you haven't already, but I'm wondering if it's an error on HubSpot's end. Are you still experiencing the issue?
This kind of error occurs once in a blue moon and gets resolved in some time. Hope it's working now as you already have tried. Still giving error, use another user's email id to log in or ask your colleague to check if the issue occurs same for him too. Then tag here a community manager, maybe they can check it with the internal team and help you out
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