"No space left on device", even tho the file explorer only shows 5 GB, of a 32 GB SD Card

[/RaspberryPi]

So my Pi 4 uses a 32 GB SD Card. Suddenly, I am being told by the system that there is no more space left for anything. 0 Byte free.
The File Manager states that the root partition has 4.7 GB, 4.5 are being used. The console command "du -h" shows me around 5 GB being used only as well.
Gparted shows the root partition having all of the 32 GB, yet only 5 GB are being used.
Using the raspi-config to expand filesystem (raspi-config > advances options > expand filesystem) or the command "expand_rootfs" have no effect either. People state that it should take 10 to 20 minutes, but the command finishes after a few seconds, after reboot no effect.
TL; DR: No online fix helps, help pls

submitted by /u/TrackLabs
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