Update failed

I’m having a problem with updating my old wordpress posts.

Today I wanted to update a post. Despite repeatedly making the changes, every time I hit ‘update’ in the editor the original text remained. I found this odd, so I updated to version 5.1

Now it seems I can publish and update newly published posts, but I cannot update older posts. I get the error message “update failed”. When I say older posts, I’m talking anything from a few days onwards.

I have tried using the classic editor. This tells me the post updated successfully, but then I check it in the editor I can still see the original, not the update.

I have re-saved my permalink settings, disabled all security plugins, and run in troubleshooting mode. None of that makes any difference. I also tried setting ‘always_populate_raw_post_data’ to -1 but again there was no difference.

Any help would be appreciated.

Here is the output from Health Check:

WordPress Version	 5.1
Plugin Versions	
 Your site has 5 plugins waiting to be updated.
 Your site has 12 inactive plugins, it is recommended to remove any unused plugins to enhance your site security.
Theme Versions	
 Your site has 6 installed themes, and they are all up to date.
 Your site has 4 inactive themes, other than twentynineteen, the default WordPress theme, and , your active theme. It is recommended to remove any unused themes to enhance your sites security.
PHP Version	 7.2.15
Database Server version	 10.0.37-MariaDB-cll-lve
PHP Extensions	
 The optional module, fileinfo, is not installer, or has been disabled.
 The optional module, libsodium, is not installer, or has been disabled.
 The required module, mcrypt, is not installer, or has been disabled.
MySQL utf8mb4 support	 Your MariaDB version supports utf8mb4
HTTPS status	 You are accessing this website using HTTPS.
Secure communication	 Your WordPress install can communicate securely with other services.
Scheduled events	 No scheduled events have been missed.
Plugin and Theme Updates	
 Plugin updates should be working as expected.
 Theme updates should be working as expected.
HTTP Requests	 HTTP requests should be working as expected.
REST API availability	 The REST API is available.
Communication with www.ads-software.com	 www.ads-software.com is reachable from your server.
Background updates	
 No version control systems were detected.
 Your installation of WordPress doesn't require FTP credentials to perform updates.
 All of your WordPress files are writable.
Loopback request	 The loopback request to your site completed successfully.