If you start seeing errors on the ftrack server you are likely reaching the inode limit on your server OS. Either increase the limit or find what is causing it. With ftrack it is likely the sessions folder that has been filling up over time as more and more sessions have been created. You can simply delete the folder "/opt/ftrack_config/data/sessions" to get rid of all the session files. All users will be required to login again after the folder has been removed. Please be careful when deleting the sessions folder to avoid accidentally deleting the data folder. 

Tip:
Instead of deleting the whole folder you can delete only the sessions that are not used. This means active users will not be logged out. This following command will delete session files that have not been used in 30 days. Please be careful when using it and make sure you type it correctly and not accidentally delete anything else.