Hi yrvin,
It does seem that your DB25 data cable is the problem. Before you buy a replacement, make it has all 25 pins connected (some comms cables don't) and there are no twisted pairs in it. Not all cables like this are the same.
On another note;
You can't use the inserts for what you want, it cuts off the signal.
You need to connect to one of the Tape Out cards using the Appropriate cable. If you don't have one or don't want to buy one, you can use AUX 1-8 outputs to your audio interface and send the required channel signal to the AUX out corresponding to the output to your interface you want. It only gives you 8 outs for recording for now.
Ideally you'll need to get the DB25 to (whatever you audio interface has) cable for each group of 8 Tape outs you have.
I think you said you only have one Tape out? That only gives you 8 outputs, so using the Aux sends would be the same. You can move the card to Tape Out 1-8 if you prefer.
I hope this helps?
Bassman
Hope