oembed preview thumbs not shown correclty
Since we have migrated our server for gnusocial.ch, the oembed thumbs are no longer generated correctly. For example on Links or video priviews, they are no longer shown.
The generated URL is e.g. https://gnusocial.ch/attachment/370327/thumbnail?w=128&h=96 - which does not work at all. The actual file would be: file/thumb/oembed-370327.jpg.
In the past the url looked something like this:
The new server has PHP 7.2 and IPv6 enabled. Besides that the config is exactly the same that it has been on our old server.
Could it be related to one of this? Is there any additional information I could provide to analyze better?