Another update :) I just tried static linking with Qt Creator based on Qt 5.11.1 (MSVC 2015, 32 bit) and it works. I can compile the application with static linking just by adding static to the config file.
Which kind of tells us, that from building qt with static linking from source everything is fine, just that something is definitely wrong with Qt VS Tools. so it kind of closes another corner for us.
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