Fester,
The only thing I can think is that when you adapted your cables together the wires that are suppose to be crossed got reverted back to just a regular 25 pin cable.
I would try if you can to just hook one null modem cable up and try that then go from there if you need a longer cable I think you can buy them in longer lengths or just get a null modem adapter for a longer standard 25 pin cable.
The other thing could be your 25 pin adapter might already be a null modem adapter causing another problem.
Just my two cents.
