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Troubleshooting with the orbeon.log

When you get an unexpected behavior, like an error message for Form Builder or when running a form, you can often find more information about what went wrong in an Orbeon Forms log file generally referred to as orbeon.log. To check this log:
Find where your orbeon.log is located. Out-of-the-box, it is written in ../logs/orbeon.log, relative to the current directory at the time you're starting your servlet container or application server. For instance, if you're starting Tomcat from it bin directory, the log file will be in Tomcat's logs directory. If you can't find the orbeon.log, or would like it to be created in a different location, in Orbeon Forms edit the WEB-INF/resouces/config/log4j.xml, locate the SingleFileAppender, and in <param name="File" value="../logs/orbeon.log"/> replace ../logs/orbeon.log with the location where you'd like the orbeon.log to be stored. Using an absolute path, like /opt/tomcat/logs/orbeon.log is often a good idea.
Update the Orbeon Forms WEB-INF/resouces/config/log4j.xml and WEB-INF/resouces/config/properties-local.xml per the development configuration. This will increase the amount of information logged by Orbeon Forms, and it is generally a good idea to keep that configuration in place for as long as you are developing forms or troubleshooting a problem.
On Linux or macOS, run tail -f orbeon.log so you can watch information appended to the orbeon.log. On Windows, you can use a tool like LogExpert (download). Reproduce the problem, at the same time watching your orbeon.log, and see if any error is being reported. If so analyze the error message, and see if this helps you find the source of the problem.
If you can't seem to find the source of the problem based on what you are seeing in your orbeon.log, when reporting a problem, it is a good idea to submit your orbeon.log with your report. To minimize the size of the file, stop your servlet container or application server (e.g. Tomcat), delete your orbeon.log, restart your servlet container or application server, reproduce the problem, and send the orbeon.log you have right after reproducing the issue.
Troubleshooting
Memory and threads