1. Scripts can and do break with new versions of mIRC if they are not maintained properly. Any scripts that are maintained should work just fine after updating. If you have scripts that don't work with newer versions than 6.21, then you really should consider better scripts.
2. Reinstalling or updating should fix whatever issue you have with the servers.ini file as long as you choose the option to overwrite the settings.
3. Make sure that the mirc.ini file is located in the same place as the servers.ini file and that the mirc.ini file is not corrupted.
Even if you get the servers working, you really should update. New versions of mIRC fix security vulnerabilities that may allow your computer to be compromised. Especially considering how old 6.21 is.