The problem was that hardware flow control was on

Now, running at 9600 baud, and everything is working as it should

Thanks for the help I have gotten.

Now all I need to find out is howto give my regular user the means of using the link And setting up the other machines

'CTS/RTS' is 'clear to send' and 'ready to send' - these are 'handshaking' signals and you shouldn't need them if you're only using a null modem cable.

You've already specified a speed of '38400' so there's no need to try any speed but that.

You do need to know what the serial settings are though:
stty -F /dev/ttyS0

If you can post the result from that, we can figure out how minicom needs to be set up.

Now Minicom usually deals with modems, so you have to configure it (and you can save the configuration) and remove all the strange modem chatter. Then you have to set the same communications parameters (speed, # of data bits, stop bits, parity, handshaking). When all that is working, just fire up minicom. You might not see the "login:" prompt when you first connect (unless minicom was running before the machine was booted) but just press the 'enter' key and it should pop up. From then on, 'minicom' should be working just as if you were sitting in front of the machine on a virtual console.

stty -F /dev/ttyS1 gives:

speed 9600 baud; line = 0;
start = <undef>; stop = <undef>; susp = <undef>; rprnt = <undef>;
werase = <undef>; lnext = <undef>; flush = <undef>; min = 1; time = 5;
ignbrk -brkint -icrnl -imaxbel
-opost -onlcr
-isig -icanon -iexten -echo -echoe -echok -echoctl -echoke

I managed to connect from one pc to another, but since then I have had problems to connect again

Once in awhile, I get to connect but then I'm not able to type in anything.. Kinda frustrating.

I had this working and did the very same thing about 3 years ago. The computers I did this on don't exist any more, I scrapped them for faster systems. I can't remember what I did. I was using Debian Sarge at the time. If I can remember and maybe find my docs on how I did it I will post it for you.