Have you check the network if all is OK and then the vmkernels?
If there is all OK then I will suggest you to check this not only in vCenter but on each ESXi host.
If there is also all OK then the next step should be to unmount all stores and mount it again or better you can fully remove it from each host and add it again as from scratch.
When this doesn't help then you have the last possibility to upgrade to the version 7 as its written in sent KB.
Alfista
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Audio-Video Accessories
Selling and Integration of Audio & Video Accessories and Technology
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