Unable to choose users and groups to assign Software Portal permissions, error failed to retrieve list. We also found a similar issue when trying to right-click on a package and choose Update Distribution Points.
This is a known issue.
This issue will be fixed in 7.6 HF5.
If you are using a previous version, you can use the work-around provided here. The updated webservicecall.js resolves the issue. It is not undergone full testing, so please apply with caution, prepared to back out should any unforeseen issues occur.
You will need to restart all consoles to get the changes to apply.
If you need to back out of the fix, delete the new one you copied in and rename the .bak to the original name, and restart services.