[imp] [IMP] Blank Compose window

[imp@amusementcity.com]

Hey,
I've installed horde 3.0.5 and IMP 4.0.3. Everything seems to work fine
except sending mails.
When I click on send button Compose window become blank and mail is not
send. I've got uploads enabled in php.ini. SMTP server is set up
correctly. I can log into it using
>>telnet "server" 25

I've run out of ideas what it could be.
My question is how could I debug what is going on after I click send
button? Is there any debugger for PHP which can do that?
Horde log file gives me no error.

Thank you for your help,
Sam