Hello Maxi,
when using the 6021/6051 address 68 is the only address that can not be used with this digital system in combination with WDP.
All addresses 01-80 can be used with the 6021/6051 except 68. The reason for this is the internal usage of this address within the program for the control of 6021/6051, because otherwise you won´t be able to control manually any locomotive with your 6021, which has been controlled by WDP before. For this reason one address had to be reserved for this release function and the author of WDP selected 68.
I think, I forgot to translate this from the german to the english manual. Sorry for this!
Please use another address for this locomotive.
Best regards
Markus