I was getting a strange set of LFD failures on my main server (most likely after some automatic system updates). The error was along the lines of  “Error processing command for line [iptables: Unknown error 4294967295], at line xxxx”
It seems that there is some collision between broken implementations on Virtuozzo (my VPS providers’ virutalisation software) and CSF (see notes on v5.77 and 5.78).
After following the manual upgrade process, everything seemed to settle down. My hosting providers later suggested that actually a clean install was the only way to sort things out fully.