This is probably caused by a PHP fatal error occurs while saving the post. The error source is one of your plugins or the current theme, so to troubleshoot, follow these steps:
- Try to enable the debug log and take a look at the log (usually in: wp-content/debug.log) to see if you can find the error.
- Deactivate all plugins and set the default theme of WordPress
- Now start activating the plugins (and the theme) one by one and every time you enable one test to check if the issue arises.
- Sometimes this is not caused by a specific plugin or theme but the website consumes the allowed memory. If the previous steps don’t help, try to increase the allowed memory size.