I have tested using the default WordPress WYSIWYG editor and did not see any errors. No one has ever reported any error with the WordPress WYSIWYG spellchecker in 3 years. Do you see an error in your BPS Security Log? If so, post that error.
To eliminate that BPS is causing the issue/problem do these steps:
If you think that BulletProof Security is causing a plugin conflict or any other issue on your website that is causing something not to work, then please use these steps below to take BulletProof Security out of the equation completely for testing. There is no need to deactivate BulletProof Security because it has a built-in Default Mode that allows you to put WordPress in a default state without deactivating BulletProof Security. If you find that BulletProof Security does have a conflict with another plugin then please check the BulletProof Security Plugin Compatibility Issues – Testing and Fixes Page to see if a fix (bypass/skip rule) is already listed. If your plugin is not listed and you have confirmed that BulletProof Security is definitely causing a conflict then please post a comment in this Forum.
1. Make a backup of your .htaccess files using BulletProof Security built-in Backup.
2. Activate Default Mode on the Security Modes page.
3. Use the Delete wp-admin .htaccess feature on the Security Modes page.
4. Test your plugin or theme.
5. Restore your .htaccess files using BulletProof Security built-in Restore.