Does this issue still persist for you? I couldn't replicate it :]

It does, yeah. I still cannot delete the calendar or timeline folders

I did some tests and it looks like there's no problem to delete any other calendars, is this still an issues and do you still want to remove that calendar?

Yes, please. And of the timeline folders, too if you could

I have also encountered this bug now. In Firefox - Windows initially, then in Edge - Windows.

I have an empty folder that has been renamed to 'random folder'. I have no child items in it but I still can't delete it. I have tried renaming the folder, adding and deleting a new child item, and moving the folder to be nested within another folder, but all delete attempts still failed.