The message was introduced in Resilio Connect v.2.12.
It applies only to macOS agents that have a TSS folder configured.
The dialog appears if a file and subdirectory from TSS folder is opened in an another application and the agent is instructed to quit.
If a user tries to quit the Agent while a file is still open, an dialog will appear. Clicking "Force quit" means that the Agent will forcibly umnount the TSS folder. The opened file may fail to be saved.
The correct solution is to save any changes in the open file and close the mentioned application(s). The If the Agent does not continue with shutdown automatically, shut it down again.
Such an error also appears for Management Console Admin, if restart of the agents or of job run on the agent is attempted while a file is being opened. Solution here is to contact the user at this Agent and inform about required restart, so that the user saves the data and closes the application in question.
After that Admin can try and restart the agent again.
In Job run the error appears and remains open for 60 seconds, after that it's dismissed automatically.
Error code SE_FUSE_APPS_PREVENT_UNMOUNT