Sometimes the serial port seems to blocked (this has happened to me with other ST terminal emulators as well), the reason for this still hasn't been discovered (probably due to a bug in the ST Bios), try resetting the terminal if this happens.
Don't use the Set RS232 Port Parameters dialog in an editor or the like, for some mysterious reason the serial port outputs a delete character if you actually change something and exit with OK(probably due to a bug in the ST Bios).
Don't try to use the underscore character in the file-selector dialogs (this is a bug in GEM, fixed in the so called Blitter-TOS).