Hi,
I managed to solve the above problem. I created a site from scratch and followed the steps in documentation and the Store Manager app worked. I guess some other installed plugins (Wordfence maybe?) were causing compatibility problems.
I’ll check if any original plugin causes the HTTP error again when I reactive them.
Thanks for your work WC Lovers!
P.D. I noticed the WCFM – Membership plugin closure of today, please kindly inform users in your website/ WP plugins site when your team would take a major action like this one.