Since October 24th, 2017 my Script Monitor has thrown consistent HTTP or Assertion Errors with no changes or issues present on the monitored site.
CA technologies has identified an issue with Script Monitors running scripts written in Jmeter 2.x. This issue occurs in rare circumstances in which the running script manually sets a cookie from a user defined variable in your script. So you will experience either an HTTP error or an Assertion Error.
NOTE: Please note this issue does not affect scripts written in JMeter 3.x.
As always, please contact CA Support if you have further questions.