When you receive an email from the support agent and click on the voting, it returns an error - the page could not be found or even a Redmine error. To fix the problem, please check the below points.
Please make sure that your ticket is not private. The option is located in the top right corner when editing/replying to a ticket.
If your tickets got permanently private when received, then you have to go to administration - helpdesk - general. Here, please disable the Create private tickets option.
Another possible reason for the error behavior is that the hostname and path are incorrectly set. Please go to administration - settings - general - hostname and path. And here, make sure that the setting is correct as it is in the example below.