Hello everyone,
Before, sorry for my poor english..
I have searched and finded a lot of discussions about my problem, but cannot find a solution for the moment.
Have two physicals DELL servers R710 (one VM by server), two DELL PowerConnect 5424 and a PowerVault MD3000i for SAN.
Connected by ISCSI.
All the structure worked very well for a while (more than 12 months), but from 2 months now, I get 10-15 errors per day. And it seems to happens more and more every day. (I haven't changed anything)
The errors come from the two servers R710 and it's related to all LUNs, all paths :
Exemple :
Lost connectivity to storage device naa.60024e80005b4437000003b34a1f5201. Path vmhba33:C2:T0:L0 is down. Affected datastores: "OrionVM".
Important : errors do not affect the VMs, all is still working...
There are no errors in the MD3000i log !
All firmewares and VMWare versions are not all "up to date", but why it worked for 1 year before errors ?
ESX version : 4.0.0, 164009
MD3000i is last firmeware revision
Selected path (for storage adapters) is fixed because of "not prefered paths errors from SAN" when in Round Robin mode.
Some pictures of config in attached files. Say me if you want more infos.
Thank you very much in advance for any help 
Christophe