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 <#4293#>[Set RS232 Port Parameters]<#4293#>
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).