I connect and disconnect coachbox and garmin connect; Nothing chances. It suddenly stopped the 29 of july 2022.
Rather than using Garmin Connect to disconnect/connect, try using the synch settings from the Coachbox site -

Then I would do the following, using only the web sites, not the apps, for each side (Garmin & Coachbox).



I just did all of this, and it is working for me, shows up on my Coachbox calendar.
It suddenly stopped the 29 of july 2022.
Has it really not been working for 8 months?