Minicom fails unless it's run with root privileges. As a normal user it fails to create the lock file in /var/lock/LCK..modem. The -o option does not override it. Is this a security feature?
Minicom is shipped without the setgid bit needed to write in /var/lock in order to minimize security risks. If you do chgrp uucp /usr/bin/minicom chmod g+s /usr/bin/minicom then any user (not just root) will be able to create a lock file.