We've had a number of reports of this, please try the following steps to resolve the issue.
Issue: Xigncode (gameguard) is disabled by your PC's other Security solutions (Norton, Avast, Bitdefender, Panda, etc).
Solution: Add Black Desert Installation folder to exclusions.
Issue: Installed files are not the newest version or are mixed with an older version.
Solution: Rename version.dat to config.filecheck
Issue: Some overlay or anti cheat software from other games can be clashing with Black Desert Online
Solution: Close any other running games/overlays (MSI / Steam overlay / Rivatuner / GeForce Experience / Skype overlay)
Issue: Error caused by proxy, VPN or Ping Boosters.
Solution: Turn them off.