When you change the repository from "stable6" to "beta6" the control panel packages are updated to the version available in the "beta6" repository. Switching from "beta6" to stable will update the packages provided that the version in the "stable6" repository is higher than the installed version. If the installed version is higher than the available one you cannot install modules in Integration → Modules. 

To avoid issues with the installation of modules:

  1. Navigate to Settings System configurationUpdate software automatically and select "do not update".  The current repository in the format  <version>-<repository>, e.g. "6.42-beta" will be added to /usr/local/mgr5/etc/repo.version.
  2. When the installed version of the control panel is available in the "stable6" repository, change the repository from "beta6" to "stable6":

echo 'stable' > /usr/local/mgr5/etc/repo.version
/usr/local/mgr5/sbin/licctl fetch billmgr --update license