I recently used a PowerShell Script to Deploy a Custom .wsp solution in the Solution Gallery and activate it.
However, when running the Install-SPUserSolution command I had this error: Install-SPUserSolution : Unable to load assembly group. The user assembly group provider threw an exception while trying to provide user assemblies for the specified assembly group.
In the WebApplication UserPolicy, add the Service Account used for “Microsoft SharePoint Foundation Sandboxed Code Service” with “Full Control” rights.
This should fix the problem.