Improperly coded or misconfigured plugins, especially misconfigured caching plugins, can break Oxygen and your website.
Disable all active plugins on your site except for Oxygen, and check to see if the problem is solved.
If so, you know the culprit was one of your active plugins. Re-enable your plugins one at a time and check for the problem after re-enabling each plugin to find the one causing the conflict.
If you get an error message in Oxygen, check your server's error_log file for an error message that is logged at the same time you see the error message in Oxygen.
Your web hosting provider can assist you with reading the error_log file.
Many times this error will have meaningful information about the cause of the problem.