Postmulti setup with Docker Lightmeter (Could not parse log line in)

I was trying out your tool on a server with a postmulti installation.
I could run different Lightmeter instances, each referring to its own log files.

I'm running docker with this command:
docker run -p 8080:8080 -v "/opt/docker_data/lightmeter_data:/workspace:rw" -v "/var/log/:/logs:ro" registry.gitlab.com/lightmeter/controlcenter -workspace /workspace -watch_dir /logs -log_file_patterns maillog_relay.hostname.net

But, the logeater doesn't seem to work, I just get a long list of:
WRN src/logeater/dirwatcher/dir_importer.go:788 > Could not parse log line in {15208 maillog_relay.hostname.net} service=controlcenter

Yet when I try it on the default Postfix log file:
docker run -p 8080:8080 -v "/opt/docker_data/lightmeter_data:/workspace:rw" -v "/var/log/:/logs:ro" registry.gitlab.com/lightmeter/controlcenter -workspace /workspace -watch_dir /logs -log_file_patterns maillog_postfix

Then it works! What am I missing here?