The server process hit the hard limit on the number of files it was allowed to have open at 5:19pm, causing a fatal exception that brought the process to a halt. This issue was detected and the server restarted at 7:53pm. The reason why the server hit this limit is unknown as this limit was raised after the same problem happened a few months ago. I'll look into why this happened again and why changing the limit does not seem to have helped.