Linux NFS and "permission denied" mount problem

Today, I have faced with following strange nfs mount problem.

[root@nfsclient ~]# mount /mymount
mount: XXX.XXX.XXX.XXX:/mymount failed, 
reason given by server: Permission denied

My nfs client is FC4 and nfs server is Centos 4.6.First, I have checked the server from nfs client side by using "showmount -e myserver" command. Exports was okay.After that, I decided to check rpc services on server with "rpcinfo -p" command.

[root@nfsclient ~]# rpcinfo -p myserver

  program vers proto   port
   100000    2   tcp    111  portmapper
   100000    2   udp    111  portmapper
   100024    1   udp    638  status
   100024    1   tcp    641  status
   100021    1   udp  32768  nlockmgr
   100021    3   udp  32768  nlockmgr
   100021    4   udp  32768  nlockmgr
   100021    1   tcp  32772  nlockmgr
   100021    3   tcp  32772  nlockmgr
   100021    4   tcp  32772  nlockmgr
   100011    1   udp    903  rquotad
   100011    2   udp    903  rquotad
   100011    1   tcp    906  rquotad
   100011    2   tcp    906  rquotad
   100003    2   udp   2049  nfs
   100003    3   udp   2049  nfs
   100003    4   udp   2049  nfs
   100003    2   tcp   2049  nfs
   100003    3   tcp   2049  nfs
   100003    4   tcp   2049  nfs
   100005    1   udp    925  mountd
   100005    1   tcp    928  mountd
   100005    2   udp    925  mountd
   100005    2   tcp    928  mountd
   100005    3   udp    925  mountd
   100005    3   tcp    928  mountd

Mandatory NFS services was working correctly as seen above rpc report.I didn't have a firewall between these machines or iptables on any of them.This was not the problem also here.I haven't check nfs client side because it was mounting other nfs server's mount points without a problem at all. So, I have focused on nfs server side. Restarting nfsd service or re-exporting nfs file systems with exportfs didn't work out. I can see nfs clients that mounted server's nfs mount by looking in /var/lib/nfs/rmtab file (showmount -a) on server. But I couldn't access /proc/fs/nfsd/ directory. It was not mounted on nfs server.So,I manually mounted it by using following mount command.

mount -t nfsd nodev /proc/fs/nfsd

After mounting /proc/fs/nfsd manually, I was able to mount the nfs server from client side again.

If you look at into /etc/modprobe.conf.dist file, you can see that when kernel installs module nfsd , it mounts /proc/fs/nfsd and /var/lib/nfs/rpc_pipefs mount points.

install nfsd /sbin/modprobe --first-time --ignore-install nfsd && 
{ /bin/mount -t nfsd nfsd /proc/fs/nfsd > /dev/null 2>&1 || :; }
install sunrpc /sbin/modprobe --first-time --ignore-install sunrpc && 
{ /bin/mount -t rpc_pipefs sunrpc /var/lib/nfs/rpc_pipefs > 
/dev/null 2>&1 || :; }