Debian installer issue

After connecting to my net6501 through a serial cable/serial-usb adapter, I 
can hit ctrl-p to stop auto booting and say "boot 81" (which is where the 
usb stick with a bootable debian image is mounted). 
The following is then written to the screen:

> |
> +
> -
> |
> |
> |
> |

Something is happening, but whatever I do next, I get:
> Undefined video mode number: 314
> Press <ENTER> to see video odes available, <SPACE> to continue, or wait 30
> sec

Does anybody know what may be going on here and how to remedy that?

Sincerely, Joh