I've been experiencing this issue as well and finally found the details and resolution to the matter. The issue (as I understand) is specific to using Jetty as your servlet container.
The cause is that Jetty will place some files in the OS's
/tmp directory--and occasionally the OS will delete them. In this scenario your admin pages will start returning 404's with the exception you've mentioned, but all searches and queries continue to execute as normal.
The solution is simple: creating a
work subdirectory underneath your
jetty.home folder (a runtime variable) will solve the problem. Jetty will see the
work folder and will place temporary files in that location rather than in
/tmp. The OS doesn't manage this location and therefore won't delete files that are still in use.
There is a bug report detailing this issue and resolution filed at: https://issues.apache.org/jira/browse/SOLR-118