Many a time, JIRA server is set up in different infrastructural paradigms. One of the most common is use of proxy server as explained here.
Some of the screens in UpRaise that are resource-intensive may take more time to load/process the request than others. In such cases if the proxy server timeout turns out to be less than the processing time 'a proxy error' is encountered. Read more here.
This error is encountered typically on JIRA on-premise version.