1) Your cron is wrong. If you want to run job every 15 mins on Jenkins use this:
H/15 * * * *
2) Warning from Jenkins Spread load evenly by using ‘...’ rather than ‘...’ came with JENKINS-17311:
To allow periodically scheduled tasks to produce even load on the system, the symbol H (for “hash”) should be used wherever possible. For example, using 0 0 * * * for a dozen daily jobs will cause a large spike at midnight. In contrast, using H H * * * would still execute each job once a day, but not all at the same time, better using limited resources.
Examples:
H/15 * * * * - every fifteen minutes (perhaps at :07, :22, :37, :52):H(0-29)/10 * * * * - every ten minutes in the first half of every hour (three times, perhaps at :04, :14, :24)H 9-16/2 * * 1-5 - once every two hours every weekday (perhaps at 10:38 AM, 12:38 PM, 2:38 PM, 4:38 PM)H H 1,15 1-11 * - once a day on the 1st and 15th of every month except December