For such type command
RDTabs.exe START SERVER 202.229.253.149 /u:user /p:password
if 202.229.253.149 is win2k server
RDTabs does not login to server

Does it work when you are in the application normally?

Yes, RDTabs does not login to 2k server from command line only.
How can I login with RDTabs from command line when password blank?

I'am sorry:)
For any win2k server RDTabs does not login to server without entering password manually (comm line or application normally)
One more time:)
How can I login with RDTabs from command line when password blank?

It should work... I don't have a Windows 2000 server to test why it's doing this, though. So have you confirmed that 2003 or 2008 servers work as expected? I'll have to go over the source code to see if I can see why...

Yes, 2003 or 2008 servers work as expected.
As I know for 2000 servers it needs to emulate key down/up from keyboard.
Two more time:)
How can I login with RDTabs from command line when password blank?

Leaving the password argument off is the only thing that is required. Not sure why it isn't working.

How can I login with RDTabs from command line when password blank?
For example we have 2003 server IP 80.32.49.1 with user administrador and password blank.
I type smth like:
cd C:\Program Files\Avian Waves\RD Tabs\
RDTabs.exe START SERVER 80.32.49.239 /u:administrador
or
RDTabs.exe START SERVER 80.32.49.239 /u:administrador /p:
and have a need to press Enter or click mouse.
Can I com-line command use instead?

I don't understand what you mean by "press enter or click mouse." So you have a username with no password, you leave off the /p argument, and it doesn't log on?

Yes, it doesn't log on when password blank.

I'm sorry, I don't have any way to replicate this error as I don't have any W2K servers, so I don't think I can give you a satisfactory answer. I don't think this will be a bug I can fix.