In one of the above answers it was suggested that sessions was the cause of running out of inodes and in our case that is exactly what it was. To add to that answer though I would suggest to check the php.ini file and ensure session.gc_probability = 1
also session.gc_divisor = 1000
and
session.gc_maxlifetime = 1440
. In our case session.gc_probability was equal to 0 and caused this issue.