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I've confirmed our config matches the section "configure secondary factor 12.1/13" on this guide: https://duo.com/docs/citrix-netscaler-alt

We ran into this also - you need to switch to the newer Duo instructions for nFactor here. Something to do with a change in the ADC code which stops the classic method working, I can't find the relevant article though. https://duo.com/docs/citrix-netscaler-nfactor

Thanks - appreciate it! That gives me something to go on.

Duo has great material