Nintex Workflow Question for SharePoint 2013:

Requirement:

Change SP Group permissions from Approve to Read for Library that has unique permissions from its site. Users in this group have to have Read permissions from Monday thru Friday and Approve on Saturday and Sunday.

Actions Taken:

I created two different workflows that use Web Call Services to change the access permissions level from Approve to Read and then back to Approve. I had scheduled these workflows to run during certain times. They both ran as scheduled, but did not change the permission levels. Instead they produced an error stating that workflow has ended unexpectedly.
Has anyone worked on a solution similar to my requirements? Any tips are greatly appreciated.