Spotlight is disabled for my main volume $ sudo mdutil -i on /
/:
Error: unable to perform operation.  (-400)
No index.


Lately, I'm low on drive space on my main volume. But after I just deleted some stuff, the error remains: 

$df -h
Filesystem                          Size   Used  Avail Capacity  Mounted on
/dev/disk0s2                       373Gi  337Gi   35Gi    91%    /
devfs                              183Ki  183Ki    0Bi   100%    /dev
/dev/disk0s4                        93Gi   60Gi   32Gi    66%    /Volumes/BOOTCAMP
map -hosts                           0Bi    0Bi    0Bi   100%    /net
map auto_home                        0Bi    0Bi    0Bi   100%    /home
localhost:/S6MhrIf1loZf9DznX1dkrf  373Gi  373Gi    0Bi   100%    /Volumes/MobileBackups
I noticed a similar error once in console.  If I recall, very similar errors start occurring regularly above 80% capacity, meaning you haven't freed up nearly enough yet.

There are several system activities that potentially eat a significant amount of space, like creating a new swap file. To protect such activities, Apple deactivates some space consuming activities, including indexing, once the drive is 80% full. 

There might be system tuning parameters which you could tweak that control this.  In fact, those errors get seen vastly more often than mdutil's errors, meaning you might learn more by opening console, identify the related errors, and google them.

I've learned to consider all those annoying log messages a "feature" myself though since leaving 50ish gigs free for temporary emergency usage has more "utility" than filling that space with infrequently used junk.