In an effort to track down what might be causing the problem, perhaps you could turn on the automatic backup prior to posting and then print (or preview) a Summary Trial Balance after each post. When you see the problem occur, please send us the backup made prior to posting.
I think it's very unlikely re-installing Ledger will change what's happening. Seems more likely to be an intermittent failure at a specific workstation, or perhaps this is triggered by a very specific sequence of steps. Checking the application log will help us track it down.
Thanks.
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Andrew Bates