I'm glad to year you're using the Sales Tax Centre in QuickBooks Online to help you produce accurate returns you can always rely on. It's important to know what each line on your return translate. I'll be happy to share more on this.
The open transactions column is similar to exceptions, but it captures differences between cash and accrual. It's normal to see an amount here if you recorded a bill and its payment in different filing periods, and you recently changed your settings between cash and accrual. This is a feature that ensures your tax reports are balanced, and you can be confident nothing will be claimed twice. Feel free to follow these steps to check your tax settings.
You can also verify which reporting method was chosen for previous returns. Here's how: Taxes > Sales Tax > Locate your filed return under Filings > Click on the drop-down beside Record Payment or View summary > Select View GST/HST-QST Detail > The reporting method used will be below the filing period.
I hope this helps to clarify things for you! Don't hesitate to reach out if you have more questions.