The worst part is that the affected players (And, perhaps, the devs) have zero idea on how the error message was triggered in the first place.
They know at least one of the causes. The festival update introduced a borked detection method to BlackCipher where the game thinks it's running in a VM (which isn't allowed) if you have a HDD mounted as an NTFS directory instead of a drive letter. The workaround was to mount the drive as a drive letter (along with its normal mount point) and use direct launch to start the game from the drive letter. That obviously won't work anymore.
8. Season 2 battles do not properly terminate when the host uses the "End Battle" button. The session acts as if the host abnormally disconnected. In addition, the player information gets mixed up and sometimes players will reconnect as the host's character that exited.