Almost always the PaperCut print-management web console, despite the Java-debugger registration.
IANA registered 9191 in 2005 as sun-as-jpda, the Java Platform Debugger Architecture port for Sun's application server, and scanners still print that name. In the field the port belongs to PaperCut NG/MF, whose Application Server serves its admin and end-user web interface on 9191 over HTTP with 9192 as the HTTPS counterpart; print servers and workstation clients also talk to it. A listener on 9191 in an office almost always means a PaperCut server.
Do not expose
It is a cleartext HTTP admin console for print accounting, user balances, and server configuration. Use the 9192 HTTPS port and keep both off the public internet; a JPDA debug port would be even worse, since JDWP grants code execution with no authentication.
$ curl -sS -m 5 -o /dev/null -w '%{http_code}\n' http://TARGET:9191/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
Get Beige Box →also available as JSON · Markdown
Corrections or a missing port? Reply @rimrocksystems.