Redwood - not able to enter a default expense account via change assignment

We have configured the Default Expense Account as key flexfield on the assignment. In responsive pages, it was no problem to add a new account combination in this field.

In the redwood page, we are not able to add any combination…not an existing combination, and not a new combination. In any case, we get the message

"Enter a valid combination of segment values."

When searching for a combination - same issue:

Config of the key flexfield: dynamic combination creation allowed