On Linux, CrashPlan PRO depends on the inotify kernel module to monitor changes to the filesystem. If real-time changes aren't being picked up or if you receive a “too many open files” error, you may need to increase the number of watches that can be created.
In the logs the error looks like this:
inotify_add_watch: No space left on device
You can temporarily update the value with:
echo 1048576 > /proc/sys/fs/inotify/max_user_watches
You can update the value permanently by putting the following value in /etc/sysctl.conf and restarting:
fs.inotify.max_user_watches=1048576