Your syntax problem may lie within your "schedule" tag (mix of month
and every, and unwanted final comma):
<schedule>1,15, of month every 2 hours</schedule>
Your "description" tag and posting text do not match the every 2 hours
in your "schedule" tag. Assuming that you want to run every 30
minutes...
Option 1: try
<schedule>1,15 of month 00:00</schedule>
and at the end of your scheduled task enqueue a task to execute 30
minutes later, if this would be within the same day.
Option 2: try
<schedule>1,15 of month 00:00</schedule>
<schedule>1,15 of month 00:30</schedule>
<schedule>1,15 of month 01:00</schedule>
<schedule>1,15 of month 01:30</schedule>
...
<schedule>1,15 of month 23:30</schedule>
This is more robust than chaining enqueued tasks kicked off by a
scheduled task, but would be more cumbersome to code, since you will
need a cron entry for each schedule.
Is this any help?