Dear Stuart,
Thank you for posting this question in Microsoft Community.
Apologies for the inconvenience caused and please don’t worry, let’s work together and try to move in the resolving direction.
Could you try to create a new notebook that have same issue or not?
If not, you could copy and paste everything over to the new notebook.
If yes, you could try to change the default browser Change your default browser in Windows - Microsoft Support
If you have tried the method with no success, I suggest you raise a support ticket from the Office 365 admin center, where the related team will collect the logs and find out the cause of the problem. Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have admin privileges and if you aren’t admin of your Office 365 organization, please contact the admin of creating a new ticket.
Thanks for your patience and understanding. Have a good day!!
Best Regards,