Compiling the latest 5.6 git branch fails with mingw32-make

When I try to compile the current 5.6 branch with mingw32-make (4.9.2, 32 bit) I get this error:

<code>
---------------------------
uic.exe - Einsprungpunkt nicht gefunden
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Der Prozedureinsprungpunkt "_ZN7QStringaSE13QLatin1String" wurde in der DLL "Qt5Core.dll" nicht gefunden.
---------------------------
OK
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</code>
In English the message should be like "procedure entry point could not be located".

The problem did not occur with older (~1 month) code (I do not know the last working set). My local system which runs in a VM has not changed.