I had a similar problem but with a different error code. I couldn't unmount the filesystem though, I had to reboot.
The problem was that the share was mounted on the client with the (default)
hard option, which means that if the server goes away, it will lock the client until the server comes back, so that the client never fails. However when the server came back, it wouldn't allow the client to reuse the old connection. I couldn't remount to get a new connection because trying to unmount would freeze the process. I think this is only designed to handle a network dropout and not a server reboot.
To fix the problem, I edited
fstab on the client to add the
soft,timeo=15 options to each NFS mount. This means that if the server can't be contacted after 15 seconds, the operation will return an error.
Now, when the server is rebooted, the filesystems all return errors pretty quickly and it's no problem to remount them. In fact, now they seem to reconnect automatically after a server reboot which is even better (possibly because I also have the
noauto,x-systemd.automount options so that they only get mounted on first use, instead of at boot time.)