Hello Laurent,
We sincerely apologize for the inconvenience.
I have checked your website and found that the issue does not appear to be related to SP Page Builder. The 500 error is being triggered by the Smart Search - Business Listings Finder plugin.
I have temporarily disabled this plugin, and after doing so, the page saving functionality is working normally from both the backend and frontend.
Could you please check the website on your end and confirm whether everything is now working as expected?
Thank you for your patience and understanding.
Best regards