Drawings of rulers (centimeter & inches) mysteriously appear in headers and footers

This is the MWE:

documentclass{scrreprt}
usepackage{scrlayer-scrpage}
begin{document}
Hello world.
end{document}

And the result:

(plus something similar in the footer)

I’m using Overleaf. I assume there is something wrong with my environment. But what? I’m compiling with pdfLaTeX from their provided 2019 version of TeXLive. Although it’s the same for 2017, and also switching to XeLaTeX in fact.

I’ve also tried clearing the temporary files, to no avail. I have of course contacted Overleaf support, but I figured maybe someone here gets a kick out of figuring this one out.