Our infrastructure provider published a post mortem on the issue we observed earlier this week : https://cloudscale-status.net/incident/59
On 2019-07-01 between 10:15 and 10:23 CEST, requests to our object storage had been answered with an HTTP error code 503. At the same time, I/O operations to bulk volumes were partially blocked as well. After 10:23 CEST, access to our bulk and object storage was fully restored.
The temporary unavailability lead to the issue that files in Drupal could not be delivered during the given time. Also newly started containers could not start because the registry was unavailable due to the partial block of access to the storage system.