Can't upload file as appuser (java sdk)

Trying to upload a file to folder of an appuser as follows:

val boxApi = BoxDeveloperEditionAPIConnection.getAppUserConnection(userId, CLIENT_ID,
CLIENT_SECRET, encryptionPref, accessTokenCache)
val folder = new BoxFolder(boxApi,folderId)
folder.canUpload("fileToUpload", 30) //here I get 403
val inputStream = fileUtil.getInputStream(filename)
val fileInfo = folder.uploadFile(inputStream,filename) // here also I get 403

The user has all the permissions, I can see it from the folder info.

I really appropriate your help.

Thanks!