LG G3 (d855) Lollipop: cannot set custom ringtone

I'm having a weird situation: I cannot add a custom ringtone to my LG G3.

I made an MP3 file with Audacity, I uploaded it using AirDroid and set it as default ringtone (like I used to do on my GT-I9300 with CyanogenMod) but it won't take it, when I receive a phone call the default LG ringt one is used. But apparently everything is set up correctly, after setting the ringtone with AirDroid if I go on the phone settings I see the correct file seleced, not LG's one. I tried playing the audio from there and it works.

To make sure it wasn't a problem with the mp3 file itself I tried:

1. cutting down the size of the file: from a full song (3M) I tried stripping it to something like 30s (1M)


2. changing format: the short file, saved as OGG instead of mp3


3. manually uploading in a different directory (instead of using AirDroid)

All tests have the same result: LG's ringtone!

What works is using one of the other default ringtones provided by vendor. If I choose one of the existing tones the setting is correctly honored.

I'm getting mad, I don't understand what I am doing wrong. I'm now running the latest version available for my phone (V20h-EUR-XX). I had Android 4.4 for a week or two, and as far as I can remember (it wasn't my priority) this wasn't working with that version either.

I found a popular AE question about ringtones and Lollipop, but sadly I cannot find the Clear cached data option on my LG.