Hi there, @justpacktravel.
Allow me to join the thread and help share additional information about running reports in QuickBooks Online (QBO).
If you're trying to open reports, we have an ongoing and reported issues with other users experiencing the same issue when trying to open reports in QuickBooks Online.
We have notified our product team and they are currently working for an immediate fix. To receive resolution updates and be added to the list of affected users, I recommend getting in touch with our QuickBooks Online Support Team.
However, if you're customizing the reports, I suggest following the steps provided by Lexiesmemere above.
Stay in touch if you have any other questions about running reports in QuickBooks Online, I'll be right here to help. Have a great day ahead!
