Registration fails, Admin Area Ticket Download fails too

Using the current version 5.10.0 on Joomla 5.4.7 an error occurs during registration:

When people try to register for an event and enter their billing information, accept the terms & conditions and check the privacy checkbox too, the system will fail after hitting the "next/send" button at the end.
A broken page will show up and tell:

array_push(): Argument #1 ($array) must be of type array, null given
(see attached Screenshot named "eb-register-2026-10-09")
The users don't get any email with tickets, but they will show up in the registrants list in the backend.
If you try to use the "download ticket" button as an administrator in the backend, the same error will show up.

array_push(): Argument #1 ($array) must be of type array, null given
(see attached Screenshot named "eb-backend-2026-10-09")

What can be done to fix this error?

In the admin backend, I already tried "tools/repair database scheme" and "tools/clear URLs", but the errors are still there.Please help, thanks very much in advance.