a problem was introduced in 1.5.7.1 that affects the substitution of $relpath$: the symptom is that, when not provided, instead of being substituted by an empty string, the string $relpath$ remains in the output (this causes the stylesheet to not be accessible). The author has accepted the patch (not by me) proposed here. I have tested it, and it fixes my issues.
Created attachment 175784 [details, diff] proposed patch
Reassigning to maintainer, CCing dev-tools herd.
Fixed; thanks...