Hi -stuartjohnstone,
Thanks for reporting the unexpected behavior you have encountered when accessing your payroll account in QuickBooks Online. There is a reported incident with processing pay run in QBO, a product investigation is being worked on for a fix with the engineers, for reference here's the investigation number INV-95416: Unable to Create Payrun "This may take few moments depending on the size of your pay run".
I would recommend contacting our Customer Care Team. That way, they can add your QBO Company ID to the affected users and you can get notification as soon as a fix has been deployed.
We apologize for inconvenience that issue has returned. Our team is now working on testing a fix which should be rolled out to production soon.