Solved.
I followed the recommendations I could implement myself, but it didn't solve the problem. However, I shared this information with the server support team:
https://www.joomshaper.com/documentation/sp-page-builder/troubleshooting#if-you-cannot-save-page-settings
The changes they made resolved the problem, but they didn't clearly explain what they did:
"We've added some security rules to the whitelist, as they were blocking functionality in the application."
Now I can save form the frontend editor. Thank you, so much.