I believe you can synchronize each calendar individually. Doing each one separately might reveal which of the two is the problem child. I suspect it's the Outlook one.

I find some conversations from people who are getting this error message when trying to sync with Microsoft Exchange Online using Thunderbird's TbSync add-on. Might that apply to you? As I read it, it has to do with Microsoft rejecting interactions that use an older version of their communications protocol EAS (Exchange ActiveSync). TbSync is still working to update their add-on to comply. Here is the conversation, which includes a link to a beta version of the add-on which supposedly fixes the problem for now:

https://github.com/jobisoft/TbSync/issues/781

Here's a link to the beta TbSync - note that it only "officially" (whatever that means) supports Thunderbird ESR 140, not the release channel 150:

https://github.com/jobisoft/EAS-4-TbSync/releases/tag/v4.18

Thanks. Disabling the Outlook calendar in TBSync and then adding it again anew seems to have done the trick. (I thought I had already tried that, but somewhere between updating TB and trying it a second time did the trick).