After FW upgrade of my routers from 384.19_0 to 386.2_2 I got this nasty message on my client side router.

Looking on the web, the primary way to get rid of this, is to do reset to factory defaults an reconfigure the router from scratch.

Finally, I've found an useful answer here:

for line in `nvram show | grep ^[^=]*=$ `; do var=${line%*=}; nvram unset $var; done; nvram commit

Please note: there are some other commands, which try to delete any variable ending with "=" character, assuming that this is an empty variable. Unfortunately, there are some variables, which are containing Base64 content (e.g. passwords) and they end with "=" character too!!!

Please note 2: Do not change the icons/assign names of/to network devices in the DHPC Server settings - they will persists in the custom_clientlist variable till next reset to factory default! (I've found there devices, which aren't on my network anymore for a long-long time)