I get this message occasionally, I am currently on 3.4.9.21. It seems to happen after environment changes, but I can't be sure of that.
Reinstalling the .msi didn't fix it (the 'repair' option), I had to completely remove all references to Janus from the project, uninstall and reinstall, and then create a new form and add a grid to the form. That reinstalled the needed references and fixed the prob.
Cheers.