I recently have a problem where I can’t delete plugins and themes I don’t use anymore because of 403 error meaning I was forbidden to access the page or folder to delete, or update a plug-in or theme. It was very frustrating. I tried deactivating all my plug-ins to see if it was a plug-in which is blocking me from deleting my plug-in. But, deactivating all my plug-ins did not let me update, or delete my unused themes and plugins. I also tried making my WP-Admin folder not password protected on the server which also did not work.
Finally, I figured out how to fix the 403 problem I was having by opening my .htaccess file on the root of my WordPress folder.
Backup your .htaccess file before you modify or delete lines on it, so if you mess something up, you can restore your backup .htaccess file by deleting your messed up .htaccess file and uploading your backup version of your .htaccess file.
When I am in the .HTAccess file, I deleted all the custom .htacess rules which plug-ins like W3 Cache Total, and Bullet Proof Security BPS security put in, and only left the text between # BEGIN WordPress and # END WordPress in the .htaccess file.
Now I can delete, and update my plug-ins and themes for WordPress since the 403 error is gone.
After you are done deleting and updating your themes, and plugins, you should re-enable and turn-on your plug-ins which make custom rules for the WordPress .htacess files to regenerate the rules in your .htaccess file.