ok, found a solution. The rewrite_rules need to be adapted in the following way:
"rewrite_rules": [
{ "source": "/WebContent/(.*)", "target": "/WebContent/$1" },
{ "source": "/(.*)", "target": "/WebContent/$1" }
]
That's what I've found a colleague did to solve this issue. However, this only fixes the 404-error, for not finding the index.html file and such. In the first js-files executed there needs to be a redirect, for this case:
if ( document.location.href.indexOf( '/WebContent' ) != -1 ) {
document.location.href = document.location.href.replace( '/WebContent', '' );
}
Not the most sophisticated solution I guess, but it does what i should redirecting before loading all the sapui-sources doesn't event slow down anything.
Thus: Solved.