FIRST: MY SERVER ALLOWS PUT OPERATIONS

I am using flutter to write a memorize app (porting the swiftUI version) and encountered the following error.

[VERBOSE-2:ui_dart_state.cc(198)] Unhandled Exception: Invalid argument (string): Contains invalid characters.:
"[{\"name\":\"user\",\"pass\":\"123\",\"udf\":[{\"colorRed\":255,\"colorGreen\":82,\"colorBlue\":82,\"colorAlpha\":255,\"name\":\"V
ehicles\",\"emojis\":[\"