Open settings file for write failed

[bphermansson]

I'm running on a ESP-01. I've checked that the SPIFFS is working with the CheckFlashConfig-sketch:

18:42:58.258 -> Flash real id:   001440E0
18:42:58.258 -> Flash real size: 1048576 bytes
18:42:58.258 -> 
18:42:58.258 -> Flash ide  size: 1048576 bytes
18:42:58.258 -> Flash ide speed: 40000000 Hz
18:42:58.258 -> Flash ide mode:  DOUT
18:42:58.258 -> Flash Chip configuration ok.

But MQTT433gateway fails to save settings from the web gui:

Open settings file for write failed!
Ideas?