Fonts table remains empty after retrieving windows

问题:

For saving ggplots with font name Calibri to pdf files, I tried to use the extrafont package in order to be able to use this font type within ggplot2.

These were the steps I took:

font_import()

After getting all the fonts (during that proces nothing seems wrong), I get this error:

Scanning afm files in <<library path>>/extrafontdb/metrics
Error in if (grepl("Bold", weight)) { : argument is of length zero

together with errors like these:

Warning messages:
1: In readLines(srcfile) :
  invalid input found on input connection 'C:<<path>>\AppData\Local\Temp\26\RtmpANG4o2/fonts/ahronbd.afm'
2: In readLines(srcfile) :
  incomplete final line found on 'C:<<path>>\AppData\Local\Temp\26\RtmpANG4o2/fonts/ahronbd.afm'

etc. 

When I run this code:

loadfonts()
fonts()

the output is NULL

and when I try to save a ggplot as a pdf using ggsave(), the error says:

Error in grid.Call.graphics(L_text, as.graphicsAnnot(x$label), x$x, x$y,  : 
  invalid font type

I guess that last error is not that surprising as the font table seems empty.

What's going wrong here and how can this problem be solved?

EDIT I thought I found the solution. When I use loadfonts(device="win") instead of loadfonts() all seemed to go well at first, but I was wrong. fonts() is still empty.