Correction: The missing fonts were from an extension. Only the fontawesome-webfont.woff and woff2 are missing (the console shows <MantisBTURL>fonts/fontawesome-webfont.woff2?v=4.6.3 causes a 404 error).

Check if this is your problem http://www.mantisbt.org/forums/viewtopic.php?f=3&t=24752

Nope. I have done a new installation. No upgrades from an old version. And as I said, it gives a 404 error (both in the console and on the network tabs on Microsoft Edge's F12 developer tools).

fonts couldn't load (as they weren't on the server, apparently)

I downloaded 2.5.1. from official download https://sourceforge.net/projects/mantisbt/files/mantis-stable/2.5.1/
The fonts can be found in folder fonts after unpacking the zip-file.

They were on the server. Re-uploaded the font files. Still the same error.

filezilla_2017-06-24_16-01-28.png (28,948 bytes)   

filezilla_2017-06-24_16-01-28.png (28,948 bytes)   

Sorry not at my dev til monday. But I had a similar issue with fontawesome
and IIS. The mime-types for file extensions woff and woff2 needed to be
registered. May give a pointer:

https://stackoverflow.com/questions/32151062/font-awesome-not-shown-in-azure-website

Ok. This bug got fixed. Had to do a bit of tweaking here and there. Thanks, @obmsch .

Thanks @obmsch for helping.

@AceOfAces it would be great if you could provide some more details for this

Had to do a bit of tweaking here and there.

Mostly trying to register the file extensions on a Web.config file.