First, is there a reason you are only moving to 6.2.1? It is also already out-dated and will not receive more updates. You should be moving to 7.2.1 or 8.0.0 at this point because they are the only supported versions.
Second, have you read all of the release notes for the newer version of Ranorex? They always include breaking changes that may cause issues. Please read those careful to make sure your tests aren't using some mechanic from 5.X that is no longer supported in the version you are trying to use.
Finally, you may need to do a CLEAN RE-INSTALL of Ranorex. To do this, do the following:
1. Uninstall Ranorex from the system.
3. Reinstall a supported version of Ranorex using the "Ranorex-7.2.1.exe" or "Ranorex-8.0.0.exe" files on the download archive page here: https://www.ranorex.com/free-trial/
Ask general questions here.