This page is organized in terms of symptom / work-around. For example,

Description: of the problem.

Symptoms The message Error: this is an error message.

Perhaps a screenshot

Solution: A description of the solution.

A good way of using this page is to CTRL-F for your error message or a portion of your stack trace.

Description: Java 9 is not supported.

Symptoms: A web UI message that says:

Server is offline
The KeYmaera X server is unavailable. All your recent work is saved (except for the click that just failed). If you run KeYmaera X locally, please restart the server.

and/or the message ClassNotFoundException java.xml.bind.DatatypeConverter in STDOUT.

Work-around: Start KeYmaera X using the command java -Xss20M --add-modules java.xml.bind -jar keymaerax.jar -launch -ui > std.txt

Work-around: Turn off windows firewall.