sdgsdgsd please follow the troubleshooting guide to retrieve the error message for the "Oops" error https://docs.flarum.org/troubleshoot it's very likely a misconfiguration of your PHP deprecation warnings output or an email server error.
For the username in previews, I'm not sure if this was implemented or not, I'll need to try the extension again. I don't use it regularly. Maybe you can share a screenshot so I can be sure what to look at?
For synopsis I don't remember testing the 2 extensions together. There's likely some logic in synopsis that ignores posts without author, or possibly there's an error if it assumes a user will be associated with every post.
