I have that error if I put RU as blocked:
CC_DENY = "RU,CN,UA,VN,BY,HK,IL,KZ,KP,SA,KR,SY,TH,TW,JP,ID,BR,AR,BD,CO,MA,SG,KR,ZW,AZ,PL,PY,SD,CI,EG,MZ,MX,PK,NP,TZ,MY,PE"
Error: FASTSTART: (CC_DENY [ru] IPv4) [] [iptables-restore v1.4.21: invalid mask `' specified]. Try restarting csf with FASTSTART disabled, at line 5790 in /usr/sbin/csf
If I remove RU from CC_DENY list and restart csf everything is fine and work well