It is possible to get more information by turning on the debug logging mode.

Turning on debug mode

1. Go to Settings > System >  Logging and profiling
   
   
   

2. Scroll down to the Default Loggers section and click on Configure
   
   
   

3. Add the Package name of the plugin and set it's Logging Level to DEBUG, e.g. com.akelesconsulting.jira.plugins.customfield.TrafficLightCFType
   

   
   

You may turn off the debug logging after you replicated the steps by

1. Go back to Logging and profiling and look for the package name in the list
2. Click on the OFF in the Set Logging Level to disable the logging
   
   
   

Getting the log files

The logs are written to a file called atlassian-jira.log which in located in the Jira Home directory.

If you are unsure of the location, you can find it from the System Info page

1. Go to Settings > System > System Info, under File Paths section, check out 
   

Annotating the logs

While replicating the issue, you can also mark the logs 

1. Go to Settings > System > Logging and profiling > Mark Logs (<baseURL>/secure/admin/ViewLogging.jspa)
2. Fill in "replication starts here" to Optional Message and then click on the Mark button
3. Replicate the problem 
4. Fill in "replication ends here" to Optional Message and then click on the Mark button again

References

• https://confluence.atlassian.com/adminjiraserver073/logging-and-profiling-861253813.html
• https://confluence.atlassian.com/jirakb/mark-logs-for-easier-troubleshooting-in-jira-server-827334452.html