I posted this on the ESRI technical support forum, but I figured I'd cross-post it here, too. I'm trying to use ArcGIS 10.6 (Desktop Advanced) on a Windows 7 Pro machine. However, something is obviously missing from my installation. When I start up ArcMap, I get a pop-up error message saying "Error in LoadPlanXML. Error number: 5 Description: Value cannot be null. Parameter name: type." Otherwise, it appears to start up and work correctly, until I actually try to do anything. Maps and data display normally.
When I attempt to use any of the tools in ArcToolbox, however, the execution always fails with error code 000210: Cannot create output. Evidently, this is some type of non-specific error. I have authorized and installed all extensions, so that's not the issue. Basically, I can view and select layers and data on the map and in the table of contents, but I can't use any geoprocessing tools at all. I tried uninstalling and reinstalling, as well as repairing the installation in Add/Remove Programs, with no luck. It's very strange.
I originally upgraded from 10.1, but to make sure I didn't lose anything important, I found all the system directories with anything in their names related to ArcGIS or ESRI and moved them to a separate archive folder. I then uninstalled 10.1 and installed 10.6, since you can't directly upgrade from one to the other. I'm guessing that wasn't how I was supposed to do it, since I'm running into this problem now.