OpenSymbol only shows rectangles instead of symbols

Hello LibreOffice community,
my son loves to experiment with fonts in LibreOffice. Some days ago OpenSymbol (one of his favorites) only showed rectangles instead of proper font symbols. He usually writes with a Raspberry Pi 3 running Raspian. I tried to find a solution in the Os configurations but i didn’t find any. So i installed LibreOffice on my Desktop Computer running Arch Linux with KDE. The same rectangles show up. I was looking around in the font configuration of KDE where OpenSymbol wasn’t installed yet. so i downloaded the font and installed it. The fontViewer DOES display the correct symbols, however LibreOffice still doesn’t. I tried the same with a notebook running Linux Mint 18 KDE with no success. The strange thing is that it actually worked on the Pi before. I didn’t install upgrades or anything that would cause problems with fonts.
Hope somebody can help me
Regards

Is your problem using the Font Viewer program to display the fonts or LibO using the font?

I can’t replicate your problem using LibO (4.3.4) with my version of Raspberry Pi3 not my desktop LibO (5.0, 5.1, 5.2). The OpenSymbol font is installed with LibreOffice and I can use Insert Special Character without problems. However, The Font Viewer program just shows rectangles instead of characters on both my copies of the opens_.ttf font. The Fontforge program shows the characters without problems. It appears my problem is with Font Viewer, not LibO nor FontForge. I hope your sun is having fun with the Pi…Peter

Thx for your response. The FontViewer is working correctly, the same is true for the font management in KDE. Just installed FontForge which does NOT show characters… Interestingly your situation seems to be reversed. However, this is LibreOffice 5.2.2.2. I will try to downgrade it and see if it works on previous versions.