Apple Calendar working on iPhone and iPad but not MacBook Pro.
Apple Calendar on MBP doesn't sync with iCloud.
iPhone and iPad have no problem syncing between iCloud and each other.
However, on the MBP's native calendar application, the calendar's sidebar shows two instances of "icloud".
Each of those instances has zero entries, and cannot be interacted with. The screenshot below shows both in expanded view (arrow pointing down instead of to the right).
Have tried all the solutions listed on the support site, and have also updated all three devices to the latest software version, but that didn't work as well.
MacBook Pro 13″, macOS 12.0