Hi,
 I created a new job (with multiple Write file tasks) on my local C: drive.  The Write file task is writing to my local C: drive (no credentials required).  I ran the job once ever minute, for about 8 hours.  I could not reproduce the error.
I  did some further analysis of the machines with the errors.  These machines have anywhere from 150 to 250 jobs scheduled with VisualCron.   On these machines we have 2 special jobs.  
1.  Job #1 - checks for jobs which have been inactivated, activated or deleted.   If any of one these 3 things happen, an email is sent and a file is written to a network share.   The trigger used is "Event - VisualCron", job Job-inactivated, Job-activated, Job-deleted).  Polling for the trigger is set at 900 seconds
2.  Job #2 - checks for a job which has a failed task but completes.   If this occurs an email is sent and a file is written to a network share.  The trigger used for this job is "Event - VisualCron, Job completed -failed, ".  Polling for the trigger is set for 60 seconds
I changed the polling to 1800 seconds for both of these 2 special jobs.  And the machines have been running without errors (ie "clean") for almost 24 hours.
Some more clarity on the errors seen with a higher polling frequency (60 to 900 seconds) are, "invalid signature", "file not found", "...attached resource not found..."
Questions:
1.  What is the recommended polling setting?
2.  What is being polled (example, a log file? or each of the jobs?)
3.  When I set the polling value to 1800 seconds, does this mean every job (?) or a log file (?) is checked every 1800 seconds?