That suggests to me that one of the form fields that is being shown is not configured correctly. This could be because a plugin you are running has added or modified a form incorrectly. Or it could be because the data in your forms is wrong.
The most common culprit is in journal locale settings. The forms rely on these to display multilingual forms, and I’ve seen lots of cases where the settings are off in the database. So first I’d recommend checking the active locales and trying to reset them if possible.
Failing that, I’d recommend using the old delete-and-test method to discover which form is causing the problem. In context.tpl, try removing the <pkp-form> tags one-by-one, testing as you go, until you can access the page without errors. That should at least tell you which form is causing the error.