When you change the repository from stable to beta the control panel packages are updated to the version available in the beta repository. Switching from beta to stable will update the packages provided that the version in the stable 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:
echo 'stable' > /usr/local/mgr5/etc/repo.version /usr/local/mgr5/sbin/licctl fetch ispmgr