Each process on a computer is allowed to open a limited number of handles to files. Resilio Connect Agent needs to trace al the files in a job, especially sync jobs, and thus may exceed the limit. As a result, this error appear, valid mainly for Linux agents.
View the current limit in the system:
ulimit -Sn //soft limit
ulimit -Hn //hard limit
To view the current number of open files by rslgent:
lsof -p <pid_of_agent> | wc -l
If number of open files exceeds the system limit, you might want to increase the limits with