…the path is too long or contains special characters…
Well, both of these are applicable in my case I guess 
I now formatted the USB stick and am copying everything onto it again to see if it makes a difference.
The folder structure in Volumio 2 looked like this after clicking music library (from Volumio’s POV)
USB
—> D0BA-303A (Volumio created this automatically, below is my own hierarchy)
—> —> Example1.mp3
—> —> …
—> —> Example5.mp3
—> —> Music (This folder does not show up)
—> —> —> Music1.mp3
—> —> —> …
—> —> —> Music5.mp3
—> —> Songs (This folder does not show up)
—> —> —> Songs1.mp3
—> —> —> …
—> —> —> Songs5.mp3
—> —> temp
—> —> —> 中文歌 (This folder and contents does not show up)
—> —> —> —> (A lot of songs and more folders with folders and songs (with special characters))
—> —> —> 英文歌 (One folder shows as “???”, I assume it is this one)
—> —> —> —> (A lot of songs and more folders with folders and songs (with abc-letters))
—> —> —> —> When I open the ??? folder, 4 random songs of this folder appear
—> —> —> 純音樂(This folder and contents does not show up)
—> —> —> —> (A lot of songs and more folders with folders and songs (with special characters))
So after typing all of this, the reformatting and copying finished, after trying there is some difference. Some observations:
→ D0BA-303A changed, this is probably just a random name given by formatting
→ Playing / adding Example1.mp3 to Example5.mp3 now works.
→ Music and Songs folders still don’t show up
→ Inside temp there is still only one folder named ???, however inside now it seems to show all songs instead of just 4 random ones
→ One of the songs that has the character ñ shows as "… seorita … " (You can probably guess where it would have been)
→ Playing songs in this folder worked once, then it did not anymore. But…
→ After that one time, I changed the three folder names to abc characters to test if it would repair the structure, but it did not.
I suspect Volumio creates a cache of the library, therefore there did not seem to be a change in the structure and the songs inside ??? also did not work anymore. (If true, this would be quite annoying, because changing something in the USB would not show up)
So I will now try to reformat and use abc characters from the start, and see if it then picks up the correct hierarchy.