Custom attribute error, but attribute exists

Hello,

Since yesterday, I am getting the following error, previously it was working and we did not changed anyting:

{"errors":[{"type":"com.personio.recruiting.api.http.FieldError","field":"custom_attribute_209068","errors":[{"reason":"errors.attribute-validation.unknown"}]}

As I can see from the documentation, this means that the attribute custom_attribute_209068 does not exists in the system. But we checked and the attribute exists.

Does someone know that else can be the problem ?