I have similar problem on my site working on OVH hosting.
CSS is not loading with the following error:
Failed to load resource: net::ERR_HTTP2_PROTOCOL_ERROR
Was trying the above solutions but neither seem to work.
The problem is with loading the file:
https://czasopisma.mage.pl/index.php/praktykapodatkowa/$$$call$$$/page/page/css?name=bootstrapTheme-simplex
The URL seems strange at least with $$$call$$$ in it, but it does work when inputted in the browser.