what caused this error?

We had a engine panic today, the hcimonitord is not able to start. the error we get is:

/hci/qdx5.6/integrator/‘s File System is too full; there must be more than 10000MB free

but at the time the file system is not full at all.  

we had to initiate a new site and copied everything over to the new site and it started to work.

so what could be the real cause of this issue? there is no error loged in hcimonitord.err or hcimonitord.log