Same here.
macOS v.12.0.1 (21A559)
iOS v.15.2
iCloud.com Browser Error-Report says
APP NAME
calendar
HOSTING APP NAME
cloudos2
CLIENT TIME
Sun Dec 19 2021 12:06:14 GMT+0100 (CET) (1639911974254)
USER AGENT
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.1 Safari/605.1.15
HOSTNAME
www.icloud.com
BUILD INFO
2204Project42__2204B36__UNKNOWN
SESSION ID
51b6ab00-****-42de-****-6949ee859263
ENVIRONMENT INFO
PROD
REPORTED ERROR TITLE
CANNOT_LOAD_STARTUP_DATA
REPORTED ERROR TYPE
CANNOT_LOAD_STARTUP_DATA
LINE NUMBER
11
USER APPLE ID
****@me.com
USER DSID
*****6534
STACK
exit@https://www.icloud.com/system/cloudos2/2204Project52/en-gb/main.js:11:921769
@https://www.icloud.com/system/cloudos2/2204Project52/en-gb/main.js:11:928019
@https://www.icloud.com/system/cloudos2/2204Project52/en-gb/main.js:11:193923
@[native code]
r@https://www.icloud.com/system/cloudos2/2204Project52/en-gb/main.js:11:461451
a@https://www.icloud.com/system/cloudos2/2204Project52/en-gb/main.js:11:562214
ERROR CONTEXT
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC is not ready. Wait for it to be ready before invoking SC.run
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: BEGIN initialize statechart
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: BEGIN gotoState: __ROOT_STATE__
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: starting from current state: ---
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: current states before: ---
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: --> entering state: __ROOT_STATE__
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: --> entering state: waitingToFinishLaunching
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: current states after: waitingToFinishLaunching
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: END gotoState: __ROOT_STATE__
Sun, 19 Dec 2021 11:06:12 GMT: INFO: SC.Statechart<sc1946>: END initialize statechart
Sun, 19 Dec 2021 11:06:12 GMT: DEBUG: Creating local CK.User object