ESXi 5.1 virtual machines no longer able to mount NFS share from other virtual machine

I'm running VMWare ESXi 5.1 as a host.

Within that host I have several virtual machines running:

NiY My main server (CentOS release 5.10 (Final))
downloadstation My download server (Ubuntu 14.04.1 LTS)
And several other which are not important

I also have a HTPC which boots the xbmcbuntu iso, using TFTP, from NiY and mounts an additional NFS share for copy on write.

And downloadserver mounts a NFS share from NiY

My problem is that since i upgraded downloadserver from Ubuntu 12 to Ubuntu 14, it can no longer mount the NFS shares from NiY. The HTPC still can mount all the NFS shares.

What I have tried so far:

Pinging all machines from all machines by hostname and IP, this works
Granting all access in /etc/exports
Disabling iptables on NiY (downloadserver is not running iptables)
Using showmount to inspect the exports from NiY, this works: http://pug205.nl/stackoverflow/showmount.jpg
Mounting the nfs share on NiY, this works
Installing a fresh virtual machine with Ubuntu 12 and mount, does not work
Installing a fresh virtual machine with CentOS 7 and mount, does not work
Mount my NAS via NFS, this does work from all machines
Checking logs on NiY, they are not mentioning any thing related to NFS
logs on downloadstation contain some information
- RPC: Registered named UNIX socket transport module.
- RPC: Registered udp transport module.
- RPC: Registered tcp transport module.
- RPC: Registered tcp NFSv4.1 backchannel transport module.
- FS-Cache: Netfs 'nfs' registered for caching
- Installing knfsd (copyright (C) 1996 okir@monad.swb.de).
- init: idmapd-mounting (/mnt/media) main process (296) killed by TERM signal
- init: statd-mounting (/mnt/media) main process (297) killed by TERM signal
- NFS: Registering the id_resolver key type
- Key type id_resolver registered
- Key type id_legacy registered
- init: failsafe main process (542) killed by TERM signal
- nfs: server {IP NiY} not responding, still trying

The last line keeps appearing in the logs

/etc/exports:

#XBMC Frodo 12.2 XBMCBUNTU
/export/XBMC/xbmcbuntu-12.2.Intel-NVIDIA *(ro,nohide,async,mp,no_root_squash,insecure,no_subtree_check)
/export/nfsroot/xbmcbuntu-12.2.Intel-NVIDIA/{MAC ADDRESS HTPC} {IP HTPC}(rw,nohide,async,no_root_squash,insecure,no_subtree_check)

#XBMC Gotham 13.0 XBMCBUNTU
/export/XBMC/xbmcbuntu-13.0~gotham_amd64 * (ro,nohide,async,mp,no_root_squash,insecure,no_subtree_check)
/export/nfsroot/xbmcbuntu-13.0~gotham_amd64/{MAC ADDRESS HTPC} {IP HTPC (rw,nohide,async,no_root_squash,insecure,no_subtree_check)

#Media share
/mnt/data/media *(rw,nohide,sync,no_root_squash,no_subtree_check)

I tried mounting with verbose logging and running Wireshark to check if anything did go to NiY (either by hostname or ip address):

root@downloadstation:~# mount -vvv -t nfs niy:/mnt/data/media /mnt/Download/
mount: fstab path: "/etc/fstab"
mount: mtab path:  "/etc/mtab"
mount: lock path:  "/etc/mtab~"
mount: temp path:  "/etc/mtab.tmp"
mount: UID:        0
mount: eUID:       0
mount: spec:  "niy:/mnt/data/media"
mount: node:  "/mnt/Download/"
mount: types: "nfs"
mount: opts:  "(null)"
mount: external mount: argv[0] = "/sbin/mount.nfs"
mount: external mount: argv[1] = "niy:/mnt/data/media"
mount: external mount: argv[2] = "/mnt/Download/"
mount: external mount: argv[3] = "-v"
mount: external mount: argv[4] = "-o"
mount: external mount: argv[5] = "rw"
mount.nfs: timeout set for Wed Jan 28 01:30:59 2015
mount.nfs: trying text-based options 'vers=4,addr={IP NiY},clientaddr={IP downloadstation}'

And it just hangs until i press [CTRL] + [C]

This is the output from Wireshark running on NiY with the previous mount command (only showing traffic from and to downloadstation)

No.     Time        Source               Destination         Protocol Info
    514 3.258878    IP downloadstation   IP NiY              NFS      V4 COMP Call <EMPTY> PUTROOTFH PUTROOTFH;GETFH GETFH;GETATTR GETATTR
    515 3.258898    IP NiY               IP downloadstation  TCP      nfs > 859 [ACK] Seq=1 Ack=121 Win=46 Len=0 TSV=314460502 TSER=78612280
   1135 35.288077   IP downloadstation   IP NiY              SMB      Echo Request
   1136 35.288140   IP NiY               IP downloadstation  SMB      Echo Response
   1137 35.288223   IP downloadstation   IP NiY              TCP      37709 > microsoft-ds [ACK] Seq=43 Ack=43 Win=2296 Len=0 TSV=78620288 TSER=314492532
   1681 63.319756   IP downloadstation   IP NiY              TCP      [TCP Keep-Alive] 859 > nfs [ACK] Seq=120 Ack=1 Win=229 Len=0 TSV=78627296 TSER=314460502
   1682 63.319769   IP NiY               IP downloadstation  TCP      [TCP Keep-Alive ACK] nfs > 859 [ACK] Seq=1 Ack=121 Win=46 Len=0 TSV=314520564 TSER=78612280
   1683 63.319790   IP downloadstation   IP NiY              TCP      859 > nfs [FIN, ACK] Seq=121 Ack=1 Win=229 Len=0 TSV=78627296 TSER=314460502
   1684 63.358905   IP NiY               IP downloadstation  TCP      nfs > 859 [ACK] Seq=1 Ack=122 Win=46 Len=0 TSV=314520604 TSER=78627296
   1944 78.359607   IP downloadstation   IP NiY              TCP      859 > nfs [RST, ACK] Seq=122 Ack=1 Win=229 Len=0 TSV=78631056 TSER=314520604
   1945 78.359639   IP downloadstation   IP NiY              TCP      [TCP Port numbers reused] 859 > nfs [SYN] Seq=0 Win=29200 Len=0 MSS=1460 TSV=78631056 TSER=0 WS=7
   1946 78.359654   IP NiY               IP downloadstation  TCP      nfs > 859 [SYN, ACK] Seq=0 Ack=1 Win=5792 Len=0 MSS=1460 TSV=314535605 TSER=78631056 WS=7
   1947 78.359715   IP downloadstation   IP NiY              TCP      859 > nfs [ACK] Seq=1 Ack=1 Win=29312 Len=0 TSV=78631056 TSER=314535605
   1948 78.359737   IP downloadstation   IP NiY              NFS      V4 COMP Call <EMPTY> PUTROOTFH PUTROOTFH;GETFH GETFH;GETATTR GETATTR
   1949 78.359743   IP NiY               IP downloadstation  TCP      nfs > 859 [ACK] Seq=1 Ack=121 Win=5888 Len=0 TSV=314535605 TSER=78631056
   1950 78.359760   IP downloadstation   IP NiY              NFS      V4 COMP Call <EMPTY> PUTROOTFH PUTROOTFH;GETFH GETFH;GETATTR GETATTR
   1951 78.359763   IP NiY               IP downloadstation  TCP      nfs > 859 [ACK] Seq=1 Ack=241 Win=5888 Len=0 TSV=314535605 TSER=78631056
   5308 95.447441   IP downloadstation   IP NiY              SMB      Echo Request
   5309 95.447488   IP NiY               IP downloadstation  SMB      Echo Response
   5310 95.447545   IP downloadstation   IP NiY              TCP      37709 > microsoft-ds [ACK] Seq=85 Ack=85 Win=2296 Len=0 TSV=78635328 TSER=314552692
  13721 138.455024  IP downloadstation   IP NiY              TCP      [TCP Keep-Alive] 859 > nfs [ACK] Seq=240 Ack=1 Win=29312 Len=0 TSV=78646080 TSER=314535605
  13722 138.455045  IP NiY               IP downloadstation  TCP      [TCP Keep-Alive ACK] nfs > 859 [ACK] Seq=1 Ack=241 Win=5888 Len=0 TSV=314595700 TSER=78631056
  16183 155.606860  IP downloadstation   IP NiY              SMB      Echo Request
  16184 155.606928  IP NiY               IP downloadstation  SMB      Echo Response
  16185 155.607035  IP downloadstation   IP NiY              TCP      37709 > microsoft-ds [ACK] Seq=127 Ack=127 Win=2296 Len=0 TSV=78650368 TSER=314612853

Solution

I've figured it out. By forcing version 3 I'm able to mount again. I'll keep looking why it is not working with version 4, but for now I'm good to go.