I have boxes instead of text when using [Character designer, Courier New, 8.0pt, everything else normal] on bits of text.
I'm on Windows 10 using FM9. Much of the text is in German, but it doesn't seem to matter if I switch the language to English.
It could be that FM9 is not Windows 10 compatible. However, it could be a font issue. Is the font bitmap, TT, PS, OpenType? Are you using a Font Manager?
The boxes usually represent unavailable glyphs/characters in a font. Are these occurring for characters outside of the first 255 of the unicode range? Try applying another typeface (preferably a Pro OTF font or a recent TT font) to see if these characters appear.
The previous writer had set up a character style with 8 pt font. I had checked Courier New on my system, but didn't notice that it does not include anything smaller than 12 pt. Thanks.