Our test environment, Ageevee.nl/agvtest, now running Joomla 5.03 and SP Page Builder 5.2.9, presented this issue. The user received an 'undefined' error upon attempting to upload images. Interestingly, users with admin rights didn't face this problem.
To address the issue, we temporarily granted the 'access to administrative interface' permission, which resolved the problem. However, it seems unnecessary for a user with an Editor role to require access to the administrative interface in the frontend. Therefore, I believe this to be a bug that warrants attention.