Hi. I am having the same issue running Studio 2018.1.2 Enterprise Edition on Win 10 Pro 64-bit. Without UiPath.Credentials.Activities the workflow loaded showed the credentials activities highlighted in red. On installing the latest UiPath.Credentials.Activities it simply failed on opening and I couldn’t access the workflow at all.
I uninstalled UiPath.Credentials.Activities to allow me to access the workflow, and then removed the problem activities, reinstalled UiPath.Credentials.Activities, and added the appropriate code elements, which then worked.
On comparison of the XAML file, the main difference appears to be 3 lines (92,96,102) where the credential activities occur. These all contain the code (xmlns:sads="http://schemas.microsoft.com/netfx/2010/xaml/activities/debugger") which when removed, the original GetAppCredentials.XAML works fine for me.
This same statement is also seen at the top of the code (line 6) and does not cause an error.
I know I have it working but I would like to know why it was a problem for me, am I missing something?