Yes Tom i did. I have 7 hosts, none of them is in maintenance mode, and i tried disconnect/reconnect etc...
So far, i managed to COLD migrate few VMs and emptied 2 hosts. i could remove these 2 hosts (ESXi4 and ESXi7) from the cluster.
Migrated only ESXi7, and i also REBOOTED ESXi4 (without upgrading it) --> now VMotion works both ways between ESXi7 ( v5.1) and ESXi4 (v4.1)!!
Then i upgraded ESXi4 to v5.1 and VMotion is still working between these 2 hosts.
My guess so far is that the reboot solved the problem partially, but this is not what i wanted!!
Julian, i am attaching my log file. Please be aware that i did many actions since the problem 1st appeared
Thank you all for your support