Hi,
I thank you for your proposal but as mentioned, I have described the environment. In my case, I do not think it would be useful to wait.
All the extensions work properly under joomla 4 and especially that the site works well with cassiopa. This seems to me a fundamental point!
I have already made several migrations of sites under joomla 4 with the same good extensions compatible joomla 4, without any particular problem (otherwise I deleted the extension), as now, with a site inaccessible!
In addition, the debug of the site seems to indicate a problem with Helix Ultimate.
In my experience on the French forums come most often templates, which is logical.
If a developer confirms that it comes from joomla, I will go back but thank you for checking with the access I gave you.
Regards