Most problems and errors you encounter while using Oxygen can be solved using this troubleshooting guide.
If you are getting a 500 error, check your server's error_log file to see the exact error message. Your web hosting provider can help you with this. This error is almost always caused by a misconfigured server or rogue plugin.
Disable All Other Active Plugins
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.
Check Your Error Log File
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.
Ensure a Modern PHP Version
Oxygen requires an officially supported version of PHP (5.6 or higher). It is rare to find hosts that provide versions of PHP older than 5.6, but they do exist.
If your host has provided you with an ancient version of PHP, you should switch hosts. PHP versions below 5.6 have not received security updates since 2016.
Clear All Caches
Clear your browser cache.
If your web hosting provider has caching enabled, clear that cache too. Your web hosting provider can provide assistance with this.
Check & Resave Your Permalinks
Go to the Settings > Permalinks page and change your permalink settings to the default.
Does this fix the problem?
If so, now go to the Settings > Permalinks page and set the permalinks page back to your desired settings.
Check to see if the problem still takes place.
If so, contact your web hosting provider.
If not, it simply means the permalinks needed to be resaved for some reason.
Re-Sign Oxygen Shortcodes
If parts of your design are missing after migrating a site, you will need to re-sign your Oxygen shortcodes.
Before proceeding, perform a full backup of your site.
Once your site has been backed up, go to Oxygen > Settings > Security in the WordPress admin panel and ensure that the "Check Oxygen's shortcodes for a valid signature before executing" checkbox is checked. If it's not, check it and then click Save Changes.
Next, click the "Sign All Shortcodes" link just below the Save Changes button.
Now, check the "I have made a complete backup of my site" box and the boxes for each post type on which you'd like to re-sign the shortcodes. Usually, you'll want to re-sign shortcodes on ct_template, page, and post.
Click the Start shortcodes signing process. You'll be able to watch the progress and see when the re-signing of Oxygen shortcodes has been completed.
Once the process has finished, check the posts or templates that were missing elements and those elements should be visible and editable again.