Infinite loop traversing NFS-mounted filesystem (QNAP server)

I have an NFS mount that has recently developed a problem whereby the mounted filesystem can't be traversed: find /media/photos/ runs fine for a while, then degenerates into an infinite loop over some file: /media/photos/2012/07/02/IMG_1180.jpg.xmp /media/photos/2012/07/02/IMG_1180.jpg.xmp /media/photos/2012/07/02/IMG_1180.jpg.xmp /media/photos/2012/07/02/IMG_1180.jpg.xmp ... The server is a QNAP TS-451+ NAS running 5.0.0.1853. The client is a Linux box running Ubuntu 20.04.3 (focal), Linux 5.8.0-43-generic. Some things I've tried, for troubleshooting: - Delete the file that find loops on. If I do that, find /media/photos does the same thing, but loops on some other file. - Run a filesystem check on the NAS. Reboot the NAS. No change. - Check the inode number, in case something was overflowing. df -i /media/photos shows 6553600 inodes total, 294671 used. That doesn't look unreasonable to me. - Run find on other filesystems. /media/photos is the only filesystem that exhibits this problem. I have other NFS-mounted filesystems from the same NAS that are bigger or use more inodes, but they all work fine. - Run find /media/photos on other hosts. - On the NAS itself, running find on the local filesystem works perfectly. - On a Mac running Monterey, find on all NFS-mounted filesystems works perfectly. - Mount the filesystem manually. Normally, /media/photos is mounted by autofs. I tried systemctl stop autofs, unmount all filesystems, then mount -t nfs -o ro mynas:/photos /mnt. Now find /mnt still exhibits the infinite loop problem above. - Check /var/log. I didn't see anything notable, but I'm also not sure what I should be looking for, beside something with the string "NFS" in it. This mount has been working fine for some years. It only failed about a week ago. This seems to have coincided with the QNAP upgrade to QTS 5.0.0.1853 (though I've been updating the client regularly as well). I'm guessing that the QNAP upgrade tickled some bug in Ubuntu's NFS client. I'm not sure how to troubleshoot this, though. And more importantly what to do about it. Some set of NFS options that I'm not familiar with, perhaps, but NFS isn't my forte.