Strange case of "MULTI: bad source address from client"

Hello,

One of my users has just had his work laptop changed, and now when he 
connects to our network via OpenVPN, his connection isn't working (seems 
up but no packet gets through), and in the server logs, we're getting 
the classical "MULTI: bad source address from client" errors. The 
strange this in his case, is that there is no IP listed between the 
brackets. The errors are as follows:

MULTI: bad source address from client [], packet dropped

And his connection does not work at all. He's also telling us that 
another OpenVPN connection, to another network, works correctly. I don't 
have any details regarding that connection though.

I suspect, despite the lack of IP address, that there is some kind of 
configuration problem on his network, like a non-functional NAT in 
certain cases, which makes his private IP "leak", but how can I 
characterize it in the absence of indication of the private IP? Or am I 
totally wrong about the possible cause?

Thanks for any help!

Best regards,

Bruno