Thank you Dave for your comment.
The FT897 is set in the VFO mode.
I tried to understand the responce on the spilt / non- split mode and found out that when I select the SPL mode in the FT897 I got the correct behaviour for WSJT. This even when I set the VFO-B completely outside the FT8 range (VFO-B at 7.15 MHZ at TX). It is ok when I press A=B and both VFO's are at the same freq. but I still have to select SPL otherwise WSJT shows "ooB" and 0,000 in red on the freq. display.
Tried a factory reset of the FT897 but still the same results, Works fine in SPL but ooB when in VFO-A mode...
btw, I don't have this issue when driving the FT897 directly without OmniRig,