UNC in MAVEN_HOME not working

Maven2 is installed in a corporate server : SERVER
From my computer, this installation is accesible on a network share : \\SERVER\MAVEN2

From a command line I setup MAVEN_HOME with this UNC:
> set MAVEN_HOME=\\SERVER\MAVEN2
And put it in the PATH
> set PATH=%MAVEN_HOME%\bin;%PATH%
When I try to build any projects, maven is unable to acces my global settings which is stored in %MAVEN_HOME%\conf\settings.xml

But when I mount a network share to this UNC:
> net use Z: \\SERVER\MAVEN2
...Setup MAVEN_HOME ...
> set MAVEN_HOME=Z:\
...And put it in the PATH ...
> set PATH=%MAVEN_HOME%\bin;%PATH%
The global settings.xml is read.

This useCase seems OK with Maven 1.

mvn -X doesn't give too any informations about the final %MAVEN_HOME%\conf which is used.
But it prints the plugin-registry.xml locations which are based on the %MAVEN_HOME%
In this case \\SERVER\MAVEN2 is badly transformed in C:\SERVER\MAVEN2 :

>set MAVEN_HOME=\\frmlefsi20\inet_dev\DEVPLATFORM\maven-2.0.4
>mvn clean -X
(...)
[DEBUG] Building Maven global-level plugin registry from: 'C:\frmlefsi20\inet_dev\DEVPLATFORM\maven-2.0.4\bin\..\conf\plugin-registry.xml'
(...)

Thanks for your support and this great tool !