Hello shutch,

What is sometimes unclear is that you can be the "someone else" using the Ledger, by having some other function (such as View accounts) open in another window.

The fastest way to get back up and running would be to restore the backup you made just prior to posting (you did make a backup - File | Backup - didn't you?). If no backup is available, contact our office to arrange a data repair.

There is a Service Pack expected for Ledger 9.0A expected sometime today or tomorrow that will improve the error handling in the posting process.