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This was resolved overnight - I am not sure how, but now the volume shows again normally.
However, it opened up a new problem with Nextcloud, which I have posted in a new thread there.
@girish & @marcusquinn thanks for the hints!
Finally the AWS guide did the trick, it was a 2 step approach (in my case):
sudo growpart /dev/sda 3
sudo resize2fs /dev/sda3
