The reason is Microsoft itself. When the local Microsoft rep "heard" (someone inside the company tipped them off), they asked to meet my team(!) and discuss the reasons for our Open Source use.

In fact, it was a meeting of 2 1/2 hours with 3 Microsoft sales/consulting reps trying to persuade us not to use Open Source (mainy they talked about "Linux" until we told them that we don't use Linux and that we don't understand what they are talking about :-) because "it is inherently insecure, unreliable" and, what was their biggest argument, "there is nobody in this country who could give you any support for Open Source", etc.

Also, they wanted(!) (actually they "required") us to tell them the reasons why we are using Open Source instead of the already introduced and long-time proven Microsoft Software in this company.

Then I started explaining the four reasons above, and when we came to the point of "Licensing Costs", they offered us TO give the Windows server licences for free.