Hello There,
50X range error codes usually point to a server side issue causing it to be unable to complete the request.
We don’t have any other reports of this so it would make sense that it is a conflict or server issue. I would start by checking for conflicts, and then contacting your host if no conflict is found.
To search for conflicts, please follow these steps.
1. First please deactivate all plugins except Optin Cat and see if the issue persists.
2. If the issue is resolved, reactivate your plugins one by one to see if the issue returns.
3. If no conflict is found among your plugins, try reverting to a default WP theme like ‘Twenty Sixteen’, or ‘Twenty Seventeen’, and see if the issue is resolved.
Head here for some more detailed instructions:
https://fatcatapps.com/knowledge-base/search-for-conflicts/
If the issue appears to be caused by a conflict with another theme or plugin, let us know, and also contact the author of the conflicting software.
If it is not, your host may have more answers. This is not something we’ve seen before.
I’ll leave this open for a while, please let us know what you find.
Regards.