PRTG file content sensor old data

We montor logfile on local filesystem of PRTG probe system and use the File Content sensor. The logfile is rotated almost daily and when the logfile rotates the search string (error of some application) cannot be found anymore. This senssor alsways worked fine but at somepoint it started seeing old occurences of the search string we monitor. These ocurrences cannot be found in the current logfile but PRTG reports the search string, it finds the string in what it looks like old logfile from several months old. Are these file's cached somewhere? Created new file content sensor on the remote probe did not solve my problem.