[Leonatus]

I tried the same font on my other computer (both Win 10, 32 bit), where I had Sigil v. 0.9.10 installed, and everything is exactly as in your example (as it used to be with my first computer as well). I installed Sigil v. 0.9.13, and everything is o.k. as well.
Whereas, on my first computer, I reloaded the font from another source, and there came the same error message. I tried it with a different font, same error.
Edit: I tried it on a different ebook with the same result. Nota bene: adding the @font-face manually, everything is all right.
And, strange enough, after having transferred the origiginal epub to my second computer with the same loaded font, the plugin works without any issue.