Repeater: Working in one template, not working in another ("Invalid value for $existingItem argument")

[MichaelHafner]

Hi all!

ProcessWire 3.0.200

1.) I use one repeater in two templates.

2.) The setup is the same in both cases: No access control for the template, no access control for the field.

3.) No problem in using the repeater via API in both templates.

BUT:

4.) In one template, I can (via Admin backend) add, edit, and delete repeater items (whether the repeater is populated or not.)

5.) In the other template, I can edit existing repeater items (whether the repeater is populated or not). I cannot, however, add new items. 

6.) exceptions.txt says: "2026-10-11 19:04:29    admin   https://vfm-online.de/cms/processwire/processwire/page/edit/?id=1063&field=person_adresse&repeater_add=1&repeater_not=1158_1253_  Invalid value for $existingItem argument (in /wire/core/InputfieldWrapper.php line 424)"

7.) But what does that tell me? And why does the error occur with *all* records belonging to this template and never with records from the other?

It used to work...  - I'm lost. Anybody who could give me a hint?

Best & thank-you

Michael