It started with thousands of "(direct) / (none)" inbound visits showing in Google Analytics.
A close look at the server log file found many lines with truncated referrer information coming from an ad server.
Our ad agency is using an adserver to display flash ads on many sites which bring the visitors to our site.
Here is good referrer information from a log file line
Here is a truncated information from a log file line
My question: What causes the referrer information to get truncated in the log file?
Is this the fault of the ad server? Is it sending http headers with truncated referrer information?
Is this the fault of my web server that in many cases writes the line with truncated referrer information?
Following the information in DerfK's answer, I have checked the User Agent information in the log file and found that the all the truncated referrer visits was by visitors that uses IE, all versions.