In addition, this error message comes as soon as I want to edit a page in the “Pages” section, but also when I want to create a new page.

Hi Peter.

We don't support SPPB 3.x and SPPB 4.x anymore, more details here >> https://www.joomshaper.com/documentation/sp-page-builder/technical-requirements#what-is-not-supported-anymore

Please download and install SPPB Pro 5.3.5

Then clear Joomla cache, and reload browser cache 2x.

Thanks for the response. Can you confirm that the pages created in the old versions are editable in version 5.3.5? After all, if they can't, a whole website would suddenly be unusable.

Thank you, this solution worked. For users with similar problem: Originally, after installing version 5.x, I first experienced a severe system failure, which made the Joomla dashboard on the back-end inaccessible for large parts. Fortunately, I had the backup. Again performed the upgrade, with an additional intermediate step:

1. make a full backup of the website Additional step:
2. remove the components older than version 2 Next:
3. install the version 5.x
4. clear the cache memory