If you are running Rankmathseo, which I currently am, the latest update to version 22.214.171.124 (and possibly 126.96.36.199) is no longer compatible with classicpress. This is the reply I got and the solution from the forum on WP.
"Thank you for contacting the support and sorry for the issue you are facing.
It seems like the new PHP functions which we used have broken the Meta Box for any WordPress version below 5.0
You can either update the WordPress to the latest version, which is very much recommended, or you can follow the following steps to install the older version of the plugin:
0. Take a backup of your website.
1. Download and install ‘Easy Theme and Plugin Upgrades’ plugin https://wordpress.org/plugins/easy-theme-and-plugin-upgrades/
2. Download Rank Math SEO v1.0.28 from here:
3. Upload it from the WordPress Dashboard >> Plugins >> Add New
4. It will automatically replace the old version with the new RC version.
You will not lose any settings or data from the Rank Math plugin and if anything goes unplanned, you can always restore the backup.
We will see if we can fix that issue, otherwise, we might have to drop the support of anything lower than 5.0 as the upcoming updates to the Rank Math plugin are aimed towards front end page builders.
You can always use the Classic Editor plugin with the latest version of WordPress to use the old editor.
I hope you understand and this helps."
The Development version seems to work okay for the moment, but not sure where this leaves users of this, which personally I was very pleased with, finding a bit improvement over yoast, if they don’t change track.
Of course in my reply to them confirming that the development version worked ok, stated I was actually on classicpress and was hoping they would continue development for us or at least maintain 4.9.8 compatibility.