Drill sometimes stops to recognize dfs directories

Drill stops to recognize dfs directories.
It happens in random manner.
this is the error.
From line 1, column 21 to line 1, column 23: Table 'dfs.root./rep/01/2017' not found SQL Query null [Error Id: 62c4f977-49d4-4bc5-8d61-9b3a1fdf2b47 on cent:31010]
The query is as follwows.

SELECT MAX(ID) FROM dfs.root.`/rep/01/2017`

I want to know root cause of the error.

Best regards