How to configure nginx/apache to deal with long page loads?

Question

PHP

There is a configuration nginx+apache+php.
When accessing a long-running script in one tab, it is not possible to open other pages of the site in other tabs. The pages pretend to be loading, but at the end a 504 error from nginx is shown.
When accessed from another browser (or from incognito mode) from the same computer, the pages of the site open normally.
How can you provide parallel access to the site when one of the tabs is busy executing a long-running script?

Answer 1

There was a similar question recently. You are blocking PHP file sessions, most likely.