Thanks. I see now that the new Windows installer has not yet been backported for FC 0.18. Therefore there is nothing to be backported.
Except of one 0.19-specific change, the PR 2630 should be backported before a 0.18.4 release because it fixes 2 issues on installing 0.18.4 over 0.18.x.
I can confirm that git commit bf27b115c0e03b8f86a84e232eb7529fdd0bff78 fixes the issue. Many thanks.
Yorik fixed it now: git commit 77550e0de38bec23af3b20be85eea38f9eb4fee7