Save/Creation 403 error after update from 1.4.9 - 1.5.9

I’ve just upgraded from version 1.4.9 - 1.5.9 following all the extra upgrade instructions for versions 1.5.0 and 1.5.1 since the upgrade I get an error whenever I try to create a new invoice or save changes to an existing invoice. I am able to view all my existing invoices and save new client details etc

console is showing a 403 forbidden error for any save calls. I’ve clearly missed a step somewhere but can’t work out what. Brand new install of version 1.5.9 is having the same problem. I’ve double checked and mod rewrite is enabled and my .htaccess file is pointing to my subfolder as required

POST 403 (Forbidden)

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