My ploblem looks rather similar, the error is the same.
TinyButStrong Error in field [a.account_aos_contracts@@sign_person_fiorp_c...]: item 'account_aos_contracts@@sign_person_fiorp_c' is not an existing key in the array. This message can be cancelled using parameter 'noerr'.
I'm sure that the field name is described correctly.
Maybe the reason is that this relation is "one-to-many"? But there is only one account to one contract and subpanel-thing is not working too. Can you help me please?