Are you sure that it was just this one message or were there two (or
more) messages alternating? I ask because *one* repeating message should
be prevented by syslog. It should print something like "message repeated
X times" instead.
I am quite sure, but I did not scroll through the 2.9 GB, I just checked
the start, some hundred lines into the file and than the last 1k lines,
always only this one message. And after that check (that took ages on a
full /) I deleted the file to get my router working again.