Server: handshake requires 'sourceDirectory', even for already configured projects

The server documentation contains "... it is enough to set “buildDirectory” ...", but the handshake

{
  "buildDirectory": "/home/daniel/Projects/nccmake/build",
  "protocolVersion": {"major": 1, "minor": 0},
  "type":"handshake"
}

results in

{
  "cookie": "",
  "errorMessage": "Failed to activate protocol version: \"CMAKE_HOME_DIRECTORY\" is set but incompatible with configured source directory value.",
  "inReplyTo": "handshake",
  "type": "error"
}