The following error appears when trying to register a site against a SharePoint environment, or when accessing Designer:
"Could not load file or assembly 'K2Field.K2Forms.Controls.Charting' or one of its dependencies. The system cannot find the file specified."
The issue was caused by a missing custom control DLL in C:\Program Files (x86)\K2 blackpearl\K2 SmartForms Designer\bin\ folder.
You can read more it in this reference.
To resolve the issue, copy the required DLLs into the folder.