I'm having problems with a particular set of assemblies using Pack and Go. When I have the "Keep full folder structure" option selected, I get the "folder does not exist...." error message. I've tried this on multiple folders on multiple drives, including brand new folders in my C: drive. I suspect the problem is related to several 3D Interconnected subassemblies I'm have in the problem assemblies. I can successfully use Pack and Go with other assemblies within the same folder, and other assemblies outside of that folder, so it has nothing to do with permissions, at least not for standard assemblies. I'm using SolidWorks 2018 SP 4.0.
A possibly related problem is once I do a Pack and Go of an assembly (call it Assembly A) with a 3D Interconnected subassembly, and I then reopen Assembly A, SolidWorks cannot find the 3D Interconnected subassembly unless I close out SolidWorks and restart it.