When trying to load a particular customer we get the "An unexpected error has occurred" page. So far I've only noticed it happening for this customer and nobody in our company can load it. I suspect it has to do with the ridiculous large number of transactions this particular customer has. However, I now need to change the email address of that customer. I can't through the UI since loading it doesn't work, and I've also tried it through a temporary script and through the editing functionality of a search. Both sadly didn't work. Is there another method that I could try? Or is there something I can do that might fix the loading issue?

Mass update or CSV import.

Try checking scripted records and turn off scripts and workflows. Then turn them. Back on 1 by 1 until one gives you the error

The CSV import worked, thanks Nick!