Problem with Cubase 5: port does not connect (analog in 4)

maybe i can get some help here. i am very experienced with computers and routings. tried everything i know. did not work.

i run cubase 5 with a tascam us 1608. all analog inputs can be routed exactly from tascam to cubase except analog 4.
the project is configured wit 9 mono inputs and 4 stereo inputs. i record drums. the analog 4 is only used, if i want to record a voice track for orientation. if i route mono in 9 to analog in 4, there is no signal. the signal appears, when i route mono in 1 also to analog in 4.

lets say, this little help awakes the 9/4 (it’s on all other inputs the same). i tried to reinstall the taskam asio driver. did not help. what could i do, to repair the obviously broken 4 analog in routing?

thanks in advance, juergen