Hi G_K_MAC,
Have you seen the changes in OneNote online? and if the isuue only occur in a specific notebook or all the notebooks.
Please Create a new notebook and then move the pages into it, see if you can sync it or not.
If the isuue remains in the new notebook, Please change another location(personal OneDrive) to create notebook and tset again so taht we can exclude the reasons fome your OneDrive.
I also sugget you changing your network environment in order to eliminate the cause from network/proxy/firewall.
Regards,
Eric