When do an internal transfer from one bank account to another bank account, Dolibarr will create two different entries. And these two entries have no link to each other, when you delete one, the other still exist. This create the problem that the debit/credit not balance.
Go to bank journal and you will find there are 4 entries, double entry for one bank account and double entry for the other bank account. and “Unknown third-party account. Blocking error” occurs to each bank account…