Configuration.yaml extra key error?

Since 0.88 I am seeing that notification that my config.yaml has “extra keys that the platform does not support”. I had a look at the line that is named, but can only scratch my head on it.
HA sais on line 48 is the evil one. But line 48 is a COMMENT (# text).
Now, from previous automations I know that the line might be off sometimes. But I honestly can not see any letter or symbol I did not use elsewhere a hundred times.
The line is within a device_tracker, specificaly checking my router for phones to see who is home.

Since the next major update will probably kill my config, I am worried about how to fix whatever is wrong here.

Does someone have any idea?

When I look thru the documentation for device tracker I don’t see hosts defined.

The sollutions are often simpler than expected.
Thank you!