I noticed the Muli italic font in my template wasn't working. After searching this forum I've found threads about this and they are related to several Rockettheme Gantry 5 templates. Although none of the threads solves the problem, it was clear that it was caused by the font files.
Cause of the problem: The Muli font that comes with the templates is an outdated version (version 1.0)
The solution: Use Muli font version 2.0 (Changes to both italics and the normal font were made)
Enclosed zip-file contains Muli version 2.0 files (regular, italic, light, lightitalic) in EOT, SVG, TTF, WOFF and WOFF2 format. Just replace the files and italic is working again.