The AD FS troubleshooting log is using more than the maximum allowed number of files, but it is not able to delete the oldest ...

The AD FS troubleshooting log is using more than the maximum allowed number of files, but it is not able to delete the oldest files. 
Directory: %2 
Files used: %3 
Maximum files: %4 

Log files will continue to be written, but they will exceed the maximum number of files. This event will recur every %1 minutes until the condition is corrected.
Retry period: %1 

User Action 
Check the permissions on the log file directory. If the directory permissions are correct, determine whether the failing file is opened by another program or whether file-specific permissions are preventing its deletion. 

Additional Data 
This error occurred as a result of a failed deletion operation on a specific file. 
File to be deleted: %5 
File deletion error: %6