Here, you will get answers to questions below:

It is possible to use an “old” vcds or vagcom kkl cabel and use it for newer bmw (2004-2011) with the software inpa?
Only short the Pin 7&8 from my vcds cable, and use it for bmw?
This is a 100% compatible interface and the explanation + schematics over the BMW mods.
It could be your interface which seems not fully following the special modifications that an OBD serial INPA/GT1 interface should have on top of a standard serial OBDII interface. You can most likely do the modifications yourselves to the interface (if you know anything about electronics and have a Volt/Ohm meter to check for Voltages and contact over the pins)
100% compatible with INPA and DIS (actually with EDIABAS STD:OBD)
Here how a true KL interface looks (work for std OBDII serial diagnosis software (not ELM type software)):
click to enlarge
Please observe the 1-4 OBD reference is for the internal connection card to the OBD socket, the schema below lists the corresponding OBD pins.
Here the mods a std OBDII serial KL interface needs to have to be BMW diagnosis compatible (works also with BMW scanner demo, would assume Carsoft would also work but have’n tried it as Carsoft doesn’t bring anything when you have INPA/NCS and GT1), as modified schematic and in words:
Here the mods it has over the std interface above:
1. pin 7 and 8 of the OBD interface connected
2. uses DTR instead of RTS (pin4 on sub D RS232)
3. Pin 1 of OBD is connected to DSR (pin 6 of a sub RS232) over a 1k resistance (for shorts protection)
4. RS232 RI (pin 9 of sub D RS232) is connected with OBD pin 16 (+ battery feed to detect battery signal) over a 1k resistance (shorts protection, the diod is polarity protection).
most INPA cables from China are copies of this interface schematic (or a similar design), it is openly published so they use it. But you can not be sure that the chinese guys did all the 4 modifications as per above, if you want to be 100% sure order the kit and build it or have it soldered by someone who can. But the easiest way is perhaps to buy one off and check the 4 points above, if they are ok the interface is most likely ok as well.
Hopes this can help someone.