In addition to this issue I've been trying this:
Template in use: Shaper_helix3 version 3.0.2
My Production site is running PHP 8.0 and Joomla 3.10.11 and it works ok.
Production site was duplicated to a new folder for migration test (still with PHP 8.0), same hosting than the Production one.
I migrated Joomla to version 4.2.5 and as I said at my initial post everything worked except when editing a SP Page Builder Pro Page (or creating a new page), I received the "Forbidden" message.
Today I've made a different test: I changed in my hosting PHP version from 8.0 to 7.4 (out of maintenance) and SP Page Builder Pro seems to work in Joomla 4.2.5 so I think there is any incompatibility afecting SP Page Builder Pro and/or Helix 3 Template with this version of Joomla (4.2.5), I repeat, I don't have problems at the Production Site with PHP 8.0 and Joomla 3.10.11.
Currently I've changed again PHP To 8.0 (I need it enabled for my production site), I've tested again at the test environment and I'm receiving again the "Forbidden message", when I debug in Chrome I see the error "Failed to load resource: the server responded with a status of 403 ()".
I've made a second test, with PHP 8.0 at Joomla 4 if I try to create a new page with SP Page Builder Pro I received the error "array_replace(): Argument #1 ($array) must be of type array, string given" in several functions, among others "array_replace(), location JROOT/libraries/src/Document/Document.php:606" or "Joomla\CMS\Document\Document->addStyleSheet(), location JROOT/plugins/system/helix3/core/helix3.php:642".
I hope this could bring you more information in order to solve this issue (I need a solution to use PHP 8.0 because we are not longer using 7.4 for security reasons).