Could not find update source

Since I first installed WPMU on almost all my client's websites, I have not been able to update any plugin. I always receive the following error: Could not find update source. I could read in the Support Forum that I am not the only one with the same issue, but haven't found any solution yet.

I am not only having this issue on this particular website or with this particular plugin, but on all my connected websites that are hosted on different servers (NameCheap, WPServeur, MediaTemple & GoDaddy), with all the plugin I have installed (Defender, Hummingbird, Snapshot, etc.).

Could you tell me what is going on with this? Can't you fix this on your hand as it seems to be a pretty common issue with your updating process?