Hi there,
Welcome to SeaTheme forum.
Did you set SSL for website? If you use of SSL certificate, please check if your ssl certificate has expired, I tried to access https, but it says there is a risk. It may cause the issue. I suggest you stop SSL for now, you can find the option on your website space host cPanel.
And it may be caused by wrong setting of htaccess file. Please read about how to fix it.
The issue could be caused by other reasons also, it is hard to say. The below link should be helpful.
https://passionwp.com/response-is-not-a-valid-json-response-error/
If you cannot fix it still, please send your website FTP account and WordPress website wp-admin access (URL / username/password) by picking up “Set as private reply”
Regards!
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