After searching for a while, I can find one explanation for this message :
(ERROR @ /home/mywebsite/public_html/core/cache/includes/elements/modsnippet/47.include.cache.php : 69) [migxResourceMediaPath]: docid could not be determined.
I've set up a Media Source called "thumbnails" with :
basePath ==> [[migxResourceMediaPath? &pathTpl=`assets/img/thumb/{id}/` &createFolder=`1`]]
So, when I browse the files tab in the left panel of the manager and I refresh the Media Source "thumbnails", then I have this message in the log. It's not surprising as there's not resources ID linked to the snippet because where in the backend.
There might be other cases like this one occurs in within the manager.
Maybe a clause in the snippet giving a default resource id would solve this, but not sure it's a good solution.