When testing manually, you probably went to the correct working directory for the script to do the editing. Starting it from any other location would most likely have caused failure as well.
- Use absolute pathes instead of relative to make sure you access the correct and existing directory.
- Include a
cd /some/where ;before your command in the crontab. Cron sets your home directory, no matter where the called program sits.
The second proposal is imho the more portable, because it does not require script changes when the location or machine is changed; you simply adapt it in your (machine-specific) crontab.