The 2.4 format configuration file accepts the option:

configsvr = false

which equates to the default, that this server is not a config server. This option is also accepted in 2.6. In 3.0, specifying this option causes an error at startup:

F CONTROL Failed global initialization: BadValue configsvr option cannot be set to false in config file

This breaks backward compatibility with older configuration files. It can be worked around by removing the option.