Hello!
When posting I am getting a 403 error, regardless of whether I select 1 or multiple sites.
The problem detected is in the number of characters.
If I make a post with a “normal” character length, it is published without any problem.
However, I need to publish 1,639,495 characters and when I try to do so, I get a 403 error.
MainWP itself does not impose a specific character limit on posts. The practical limit depends on various factors, such as the database configuration, server settings, and your hosting environment.
Have you tried creating that post with 1,639,495 characters directly in the WP Admin of a Child Site without using MainWP?