error "502 Bad Gateway nginx" when trying to access the panel

When I want to enter the panel, an error 502 bad gateway nginx appears like this image.

But fortunately the applications on the vps are still safe. Can anyone help?

Try executing this command to fix it

rm -f /tmp/update_to7.pl && curl -k https://node.aapanel.com/install/update_7.x_en.sh | bash

The problem was solved after I restarted the server.
What do you think caused this?