Another problem that the converter is not able to handle is an invalid OpenVPN configuration file -- for example, one which refers to certificate or key files that do not exist.
For example, you have a configuration file that contains a line like:
ca ca.crt
and you move or copy that configuration file into a subfolder but do not also move or copy the "ca.crt" file into the same subfolder. In that situation, neither OpenVPN nor Tunnelblick will be able to find the "ca.crt" file, because they will both be looking into the subfolder. So OpenVPN will not allow you to connect using that configuration, and Tunnelblick will not be able to convert it to a Tunnelblick VPN Conversion.
The workaround is to move any such invalid configurations out of the "Configurations" folder (to the Desktop, for example), and retry the conversion with only valid configuration files in Configurations or its subfolders.