I've experienced the same problem (also reproducible) since version .50 (got up to .59 and went back to Jon's .43 LTS)
If I turn iptraffic without first disabling NAT acceleration (as pointed out by BillyBobX) the router will crash on restart.
Before that version, it would automatically disable NAT acceleration once you activated iptraffic, so something might be wrong in the detection mechanism.
Again, working fine for me. I enabled IPTraffic on an RT-AC87U. It rebooted, and once booted the Tools -> Sysinfo page reports that CTF is noew disabled because it's incompatible with IPTraffic. I also reviewed the code that handles the automatic disabling of CTF, and that code is quite straightforward in what it does and how it does it. It's just one single call to check if cstats_enable is set to "1".