If you receive a browser error page that contains the following messages:
javax.servlet.ServletException: Could not parse specification context:/WEB-INF/Error.page.
java.io.FileNotFoundException: /opt/papercut/server/tmp/webapp-8196/WEB-INF/Error.page (Too many open files)
or see the same thing in the server.log file, and your PaperCut server is running on Linux, you may be encountering a resource ulimit for the PaperCut user account.
Many Linux distributions limit the number of open files/sockets to a number acceptable for a user in a desktop session. PaperCut is a server application, including a web server and database amongst other services, and may need more resources than the default limits allow.