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.
Plugin Conflict Test
Improperly coded or misconfigured plugins, especially misconfigured caching plugins, can break Oxygen and your website.
To run a plugin conflict test, 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. you can then 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 (7.1 or higher).
If your host has provided you with an ancient version of PHP and you're not able to update it, you should switch hosts.
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.
Regenerate Oxygen's CSS Cache
If your site has been migrated, you will likely need to regenerate Oxygen's CSS cache via Oxygen > Settings > CSS Cache.
The most common cause of a 500 error on an Oxygen site is that your site has run out of memory. If the error is only occurring when you try to edit a template, the issue may be caused by the number of items that are being previewed by the template. You can reduce the number of items in the preview dropdown box by setting the Preview Dropdown Limit to something like 10 via Oxygen > Settings > General.
An unsupported database collation can cause Oxygen's shortcodes to be created incorrectly. You will need to make sure that your database and tables are encoded with utf8mb4_general_ci or utf8_general_ci. If the database is set to UTF8 encoding and not utf8mb4_general_ci, 4 byte characters (aka Emojis) in the post meta will prevent anything from being saved.
If you're not sure how to change the database collation on your site, please contact your web host.