Usually when this happens, the content that’s trying to be saved is rejected by the server.
The most common way of trying to find out if it’s a Rey issue, hosting issue or other plugins issue, is by disabling plugins, one by one until the culprit is found. If by disabling plugins the issue still occurs the problem might be a server related problem and you could try reaching out to your hosting support to check the error logs.
You might also want to try removing elements from the page and see if there’s a particular one that causes the error.