Is "Webtop error" a Safari / Mac related error or a website error?

Hello Everyone. I have an email address with my cable/internet provider Optimum. When I try to delete the messages in my trash bin, (whether individually, or in bulk) I receive a "webtop error" message. (I did the usual delete history, check for updates, reboot, etc.) I'm pretty sure it's a problem on their end - but I called and got nowhere with the rep.

Wondering if it's something I can fix on my side. Thanks!