opening forge.codesys.com, my browser fails to decode the downloaded font from https://forge.codesys.com/_ew_resources/theme/ownforgetheme/fonts/Roboto-Regular.ttf

The reason, is that this file is not really a ttf file, but a html file for the github page that this font was downloaded from.

Solution, is to replace the file at the above forge url, with the correct file which can be donwloaded from this url:
https://github.com/google/roboto/raw/master/src/hinted/Roboto-Regular.ttf

Note that if you replace "raw" with "blob" in the above guthub url, you will get the false html-ttf document that forge.codesys.com is currently serving.