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I'm sorry to hear that you've encountered errors when trying to respond to a case. If the case is still open and you've tried the cookies & cache clear the way April suggested, please try calling customer service to both report the issue and to get a response added to your case.
Calling is a more immediate conversation than emailing, which can be crucial when meeting dispute deadlines. Click Contact at the bottom of the PayPal website, then click Call us, and log in or call as a guest if there is any issue logging in.
I hope this can be resolved quickly!