Check the log file at '/usr/local/flamenco/instances/ChEMATIC_-_19-09-06/log.txt' for more details. Here are the last few lines:
Loading plug-in: WebUtils at /usr/local/flamenco/webware/WebUtils
Port 21600: [Errno 98] Address already in use.
Port 21601: [Errno 98] Address already in use.
Port 21602: [Errno 98] Address already in use.
Port 21603: [Errno 98] Address already in use.
Port 21604: [Errno 98] Address already in use.
Port 21605: [Errno 98] Address already in use.
Port 21606: [Errno 98] Address already in use.
Port 21607: [Errno 98] Address already in use.
Port 21608: [Errno 98] Address already in use.
Port 21609: [Errno 98] Address already in use.
Port 21610: [Errno 98] Address already in use.
Port 21611: [Errno 98] Address already in use.
Port 21612: [Errno 98] Address already in use.
Port 21613: [Errno 98] Address already in use.
Port 21614: [Errno 98] Address already in use.
Port 21615: [Errno 98] Address already in use.
Port 21616: [Errno 98] Address already in use.
Port 21617: [Errno 98] Address already in use.
Found an available port: 21618.
Listening on ('127.0.0.1', 21618)
Creating 10 threads..........
Ready (0.11 seconds after launch)