Thank you for visiting the site to check. The error does not occur because I have currently reverted to version 3.2.0 of the Metabox plugin on the site. When I update to 3.3.4, the custom fields disappear.
Please update Meta Box AIO to the new version 3.3.5, then clear all caches: plugin, browser, hosting, Cloudflare (if you use it) ... and recheck the issue.
On this site we are not using Meta Box AIO, this client only uses Meta Box Lite. I got installed Meta Box Version 5.10.19 and Meta Box Lite Version 2.3.3. We need to fix this se we can keep developing the site. What should I do?