Along the lines of this older post I saw that nlbwmon stops responding after a day or so.
Symptoms:
I had already restarted the service before finding that post, so no trace today.
CPU usage wasn't anything special.
Other than tracing next time it happens, anything else I should do / add ?
Which version was installed/running exactly? The next time this happens, please try to capture an strace log using strace -f -p $(pidof nlbwmon)
I'm using OpenWRT 18.06.4, so it's version 2019-06-06-4574e6e8-1
I made a note to look at this post next time it happens.
strace: Can't stat '32469': No such file or directory
However, strace -f -p 25006 gives an endless sequence of:
recvmsg(7, {msg_namelen=12}, 0^C = -1 EAGAIN (Resource temporarily unavailable)
(I was testing IPV6 prefix assignments and I restarted the network a LOT)
Hmm, today I rebooted the router multiple times to test ipv6 changes and now the db is gone 
Any tips?