Hmmm.... So, it happened again, for another user. There was a valid "not initiated" basket in the backend, but no way to reach that basket from the frontend for the user. No redirection for that basket.
I also tried making a new subscription, but I was not getting to the existing basket. Instead I was shown a new basket.
I clicked on cancel and after that both baskets were gone, the old and the new one.
Question: Could it be a time issue? My baskets are valid for a very long time, about 1 month. Could it be that the redirection after login does not work any more after some time? Maybe the redirection stops working after 1 week or so, but my basket is still valid?