Thanks for reporting, this sounds like a bug in Pixelbox. I will have a look.
When creating a new slice config, Pixelbox will attempt to create a file slicesConfig.json in the project”s assets directory. It might be this part that fails for some reason?
Do you see this file in the assets directory? If not, you can try to manually create this file and edit its content with the following:
{
"_type": "Nine slices config",
"sprites": {}
}
Then try to close and reopen the project.