CSS files added with %5C in path

Problem/Motivation

There is something wrong with the embedding. I used the subtheme without SASS and it adds %5C to the URLs. There are also some dots in the paths... no idea where this comes from.

<link rel="stylesheet" media="all" href="/core/../themes/contrib/bootstrap%5Ccomponents%5Cbreadcrumb%5Cbreadcrumb.css?sjfvo1" />
<link rel="stylesheet" media="all" href="/core/../themes/contrib/bootstrap%5Ccomponents%5Cmenu_columns%5Cmenu_columns.css?sjfvo1" />
<link rel="stylesheet" media="all" href="/core/../themes/contrib/bootstrap%5Ccomponents%5Cmenu_main%5Cmenu_main.css?sjfvo1" />

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes