"Storage Space Running Out" even though plenty of storage remains?
As Alexandar already pointed out, there are different partitions involved. The "400MB on your system partition" (actually, there's only ~ 300 MB free) have no effect here at all, as you cannot use them: /system is mounted read-only during normal operation, and mostly reserved for the OS and its core apps.

Let's walk your tree one by one:


/dev, /system, /efs, /cache, /radio: these are "special locations" used by the system, and not available (directly) to your apps and your data.
/mnt/secure, /mnt/asec, /mnt/obb: similarly.
/data and  /storage/sdcard0: your SD cards are not related to that error message.



  
  /datadata: here we go, this is the troublemaker: just 26.4M are free here. The error you're describing is triggered as soon as free space drops below 25 MB (or below 10% sometimes, which would be ~40 MB). No matter how much space is available elsewhere.
  


For actions to be taken, take a look at the insufficient-memory tag-wiki, which explains some more background and offers first-aid as well as links to related sources.