EmailSend doesn't work on Windows 8

On windows 8 the command EmailSend returns the error: Invalid Addresses

The very same database-, record- and script-combination works just fine on windows 7, so it must be a problem with Windows 8

I am using FMP(A) 12v4 with JRE 1.7v25 and Email-plugin version 1.972 on Windows 8 professional.

I've tried this in 2 different network where there are an exchange-smtp-server and a postfix-smtp-server. Using any of these servers result in the same: it works on Windows 7, but not Windows 8

I allready had contact with support about this issue and I thought it was solved, but it seemed to have worked only once and now I get the invalid addresses error again. For now my customer only uses Windows 7, so there's no pressure yet, but I am running Windows 8 and must assume the code I write just works cq have to fire up an old windows 7 pc to check if it works, before I deploy my solution to the customer.