As reported by Jimmy Bergmark, AutoCAD 2017 SP1 will break add-ins that use Autodesk’s built-in autoloader mechanism. It looks like it’s a problem caused by third party applications, but it’s not. It’s entirely Autodesk’s fault. The only fix at this stage is to uninstall SP1.
It’s astonishing that Autodesk would release a service pack like this, introducing a nasty bug that will break customers’ existing functionality. This reminds me of the comedy of errors that was AutoCAD Release 13 with its multitude of updates, many of which introduced new bugs as well as fixing others. AutoCAD 2017c4a, anyone?