Android mkdir() returns false, doesn't create folder

This is driving me crazy. I have tried all kinds of syntaxes but both mkdir() and mkdirs() return false.

My code:

String extStorageDirectory = Environment.getExternalStorageDirectory().toString();

File folder = new File(extStorageDirectory, "myportal");
boolean bool = folder.mkdir();

File pdfFile = new File(folder, fileName);

try{
    pdfFile.createNewFile();
}catch (IOException e){
    e.printStackTrace();
}
FileDownloader.downloadFile(fileUrl, pdfFile);

I was getting and IOException: No such file or directory when trying to create the file. The logcat part showed me that no directory was created:

Log.d("BG", "URL: " + fileUrl);
Log.d("BG", "pdfFile: " + pdfFile);
Log.d("BG", "Ext Storage: " + extStorageDirectory);
Log.d("BG", "Ext storage state: " + Environment.getExternalStorageState().toString());
Log.d("BG", "Mkdir return: " + bool);
Log.d("BG", "IsDirectory: " + folder.isDirectory());

And what is printed:

05-26 22:43:03.797 19364-30646/com.kristmiha.myportal2 D/BG: URL: http://192.168.100.65:80/myportal/upload/orari.pdf
05-26 22:43:03.798 19364-30646/com.kristmiha.myportal2 D/BG: pdfFile: /storage/emulated/0/myportal/orari.pdf
05-26 22:43:03.798 19364-30646/com.kristmiha.myportal2 D/BG: Ext Storage: /storage/emulated/0
05-26 22:43:03.804 19364-30646/com.kristmiha.myportal2 D/BG: Ext storage state: mounted
05-26 22:43:03.805 19364-30646/com.kristmiha.myportal2 D/BG: Mkdir return: false
05-26 22:43:03.805 19364-30646/com.kristmiha.myportal2 D/BG: IsDirectory: false

I've double checked permissions and I've put them in the right place. I think I read somewhere that after KitKat we are not allowed to write in the external storage, but have found no solution yet.

Creating external storage directories requires the WRITE_EXTERNAL_STORAGE permission; without this permission, attempts to write external storage will fail. Also, not all directories are necessarily writable; use getExternalStoragePublicDirectory() to get a directory that has shared write access (other external directories may be read-only). However, if the purpose of writing to external storage is to share the file with other applications, you should strongly consider using the FileProvider API, instead (see also Setting Up File Sharing); with this strategy, your app stores files in its own, internal app-specific directories, but then enables selective sharing of these files to other apps through a content provider. This strategy provides greater security for the files and also makes it possible for you to provide greater access control over reading/writing of the files.

Since you say that you already have the required permission*, most likely where you are getting things wrong is in the call to toString(). There is no guarantee that the toString() method on a file returns its full path. Use getPath() or getAbsolutePath() when concatenating these. It is also advisable, when choosing to write to external storage, that you first check its state; the external storage can in some cases be ejected/unmounted and not available.

*You should verify this with ContextCompat.checkSelfPermission(). You should add a call to requestPermissions() if the permissions are not present.