Either there has been some corruption in the data, or someone has posted a journal to the control account. Back up first and add "error" to the backup name.

Go to the Nominal Ledger, Debtors Control Account (usually 1100), Activity screen.  If you know it's a recent error you can filter for just the last few months, otherwise let it display all transactions.  Export it to Excel.  In Excel, sort it into transaction type order. 

You are looking for transaction types other than S*, so it could be a BP or JD for example. If it's a journal, make a note of the details then delete it.  Depending on why the journal was posted in the first place, you may need to recreate it using a different N/L code.

You mention that Sage says you can't post to nominal code 1100.  You can, but it comes with a message warning you that it won't be reflected in the ledger. It can be overridden but only do so in exceptional circumstances.

It may be corrupted, sage wont even let me change or delete it that way either. 

We have no idea what it even goes to or why it was posted. 

There are not details of associated transactions, no account reference or any information like that. 

It may be corrupted, sage wont even let me change or delete it that way either. 

We have no idea what it even goes to or why it was posted. 

There are not details of associated transactions, no account reference or any information like that.