Thanks for the quick help.
Unfortunately that was not the cause of the mistake ...
The call looks like this:
<? php if (($ s5_fonts_highlight! = "Arial") && ($ s5_fonts_highlight! = "Helvetica") && ($ s5_fonts_highlight! = "Sans-Serif")) {?>
<link rel = "stylesheet" type = "text / css" href = "//fonts.googleapis.com/css?family=<?php echo str_replace (" ","% 20 ", $ s5_fonts_highlight); if ($ s5_fonts_highlight_style! = "") {echo ":". $ s5_fonts_highlight_style;}?> "/>
<? php}?>
According to my browser, however, "only" the following (incomplete) address is called
https://fonts.googleapis.com/css?family=