Henry’s got the solution right. Here are the details of what you’ve run into.
Every account stores a number to be assigned to the next transaction to be added to the account’s ledger.
This “next transaction number” starts at zero when the account is created.
Every time a ledger item is added to the account (either directly by adding an item to that ledger, or by posting an item from another account) the counter is incremented by one and the transaction is ‘stamped’ with the new number.
The number maxes out at 64k – once the next transaction number reaches that point then no further transactions can be added to the account.
The next transaction number is *NOT* decremented when you delete or purge transactions.
So, why would you get that error if there are only 70 or 8,000 ledger items in it? It’s because over the life of that account 64k items have been added – whether they are still there or not does not matter. Look at it this way: Say you you create a new account. Then you insert a ledger item and then delete it. Repeat the insert and delete 64k times and you would get the ledger item limit exceeded error even though the account is empty.
Fixing it, as Henry suggested, it straightforward. Backup the books and then run Reorganize Books. In this process all accounts are newly created (with the next transaction number at zero) and it will only be incremented as many times as there are ledger items added to the account when the books are rebuilt.
The end result will be you account’s next transaction number will be reset to whatever count of items are currently in the account (70 or 8,000) and you can continue posting to/from the account.