Notifications in Admin Bar no longer working

Hi all,
my WP site is fully updated incl. PHP. lately however, the orange notifications button on the top right corner of the admin bar does not work at all. I am getting comments and follow ups but the notifications are all stuck on an endless loading icon when I click the button.

Image to the broken feature:

This used to work fine. I cannot find any admin bar setting under my WP admin tools either.
Anyone know how to solve this?

many thanks

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