I would start by forcing a rebuild of your normal.mxt file. This is a base template that is generated for every ArcMap user and is what the application uses to define your viewer on launch. Navigate to the path below and delete the normal.mxt file located in the folder. Re-launch ArcMap. Your viewer will look brand new as if its first time it's ever been used and a new normal.mxt is built into the folder. Try adding back in the toolbar after all this.
C:\Users\<user_name>\AppData\Roaming\ESRI\Desktop<version#>\ArcMap\Templates
Based on latest pages it is supported out through 10.5 so if other users in your shop are using the same installer package you are and it's working fine then, in theory, you are set but still may be worth re-downloading the latest installer and running from it if that hasn't been done. Attribute Assistant | ArcGIS Solutions
Not the same error but potentially something else to watch for https://community.esri.com/thread/179305