I'm getting the same error and my drive doesn't look anything close to being full. I'm running 6.8.3 nvidia and this ONLY happened once I upgrades to 6.8.3.
this is on a 16GB cruzer fit
What got it going again was to reboot via SSH and notice that a few dirs are now smaller.
crazy that just 6MB kept the web gui from loading while the drive has gigs of space available...