I wanted to confirm same that I cannot open the preference folder using AF button. This is a on Kubuntu. Here is from the log from Player.log https://pastebin.com/1r2qHZXj
The last couple of xdg-open: unexpected argument 'Forge' lines occur every time I click on Open Preferences Folder button. Looks to me that you space in the preferences folder name throws it off, ~/.config/unity3d/Kenney/Asset Forge
P.S. The lines above the xdg-open error, occur when I launch Asset Forge.
xdg-open: unexpected argument 'Forge'
Try 'xdg-open --help' for more information.
xdg-open: unexpected argument 'Forge'
Try 'xdg-open --help' for more information.