Our Technical Team are looking at this right now. If you haven't already we suggest that you submit a support ticket in your Portal, so that we can contact you about it.
Our Engineers are investigating reports of MoodleCloud users attempting to access courses receiving the 'Course ID Invalid' error and being unable to access that course.
Update 2021-09-08 15:30 GMT+0: Engineers have confirmed issues with some MoodleCloud sites in all regions related to https://tracker.moodle.org/browse/MDL-72402 and are investigating root cause.
Thank you Kelly for your reply and for investigating the issue. I have submitted a support ticket now (I was looking yesterday for how to do that and couldn't find it so easily. I found out now but where to submit the support ticket is not so obvious).