Sorry, I don't know why but it worked.
I unpublished the document and then published and it worked.
It appears that documents that have not gone through this process can now be previewed.
Perhaps it takes a while for the preview to become available?
Thank you for your reply.
The problem is currently not occurred, so I can't confirm if the network was related.
If the same problem occurs, I'll take a closer look at the network at that time.