Sorry, was very busy.
So, you're updating from build 151018 to 151126 on an ISS server with MSSQL, right ? Many people did the update from 151018 to 151126 without encountering problems. So it's probably related to ISS (ISS and MSSQL: why ???

).
Could you please provide me the exact error message please ? Normally, the comfortUpdate should check the permissions before doing any change, and should ask you to change those permissions if needed... So you maybe found a bug.
For now, if it's an emergency for you, you can still update using this package :
comfortupdate.limesurvey.org/old_updates...te-151018-151126.zip
Just unzip it, and copy paste its content to the root of your LimeSurvey installation (of course, backup it first). Then, change the build number to 151126 in application/config/version.php