SAP Note 10879 - FB02 document change after closed posting period.

Symptom

You can change document fields that are entered in table TBAER even
if in the document change rules it was specified that changes are no
longer possible upon closing of the posting period.

Cause and prerequisites

in the posting period default setting, only the masked entry with '+'
for all account types.

Solution
For all account types (S,K,D), you have to make a separate entry and
specify the account interval in the posting period table.

Key word: FB02