Software update required results in a more meaningful message.
D100 installation manual page 6-15 - "Step 1 Configure DSAB" covers this.
Sorry - don't have the D180 manual but it will be in there.
Yours sounds like something more fundamental.
Try turning off the AP74 and servos in turn and try configuring DSAB again to see what if anything changes - it would narrow it down a bit.
You haven't got your DSAB A and B crossed over somewhere, have you? Just an idea!
And yes, you can have a D180 + AP74 with no servos: it's not a factory supported configuration but I have been using a D100 + D120 + AP74 config for the last 3 years without problems!