Playlist skips local songs

[FIevo]

Plan

Premium

Country

Netherlands

Device

Samsung Galaxy S5 Neo

Operating System

Android 6.0.1

My Question or Issue

 I recently got 3 months of Premium for €1, and have been using Spotify for my daily commute playlist since. I have recreated the playlist in Spotify, but I keep having weird issues with the 2 local songs in the playlist being skipped.

The following cycle has repeated itself 3 times already since I started using Spotify:

Day 0: I re-install Spotify on my PC, laptop ánd phone, then synchronize my playlists on my phone. I download all songs in my daily commute playlist, then play it and skip through all songs to verify it works. No issues at all

Day 1: On my way to school, the playlist plays without any issue. On my way back home, the 1st local song plays just fine, but the 2nd one is skipped. And for some reason, the song áfter the 2nd local song (which is a regular Spotify song), is skipped as well. Yet for some reason, I can play the skipped local song when I manually tap it to start, and the playlist continues without any issues

Day 2: The 1st local song is now also being skipped, and the 2nd local song is still being skipped as well. I can start both of them manually by tapping them after which the playlist continues as it should, but Spotify won't play them automatically

And some additional notes:

- When I look at the playlist on my phone, both local songs have the green arrow icon, indicating they are downloaded. Yet both songs are also greyed out, but I can still play them manually

- My local songs are stored in a folder in my Dropbox, to ensure all my devices can access them

- My devices are always on the same wifi-network, except my PC, which is connected through Ethernet. I've already synced my songs between my laptop and phone once, same issue

- I have already tried re-installing Spotify and clearing the cache several times on each device

- The playlist plays without any issues on both my PC and laptop, but not on my phone (the one place it has to work properly)

I also found this topic already which appears to describe the exact same problem on the exact same device. This, and the fact the playlist plays without issues on my PC and laptop, makes me believe it's somehow an issue with my phone. This user 'fixed' it by switching to a different phone, but that is not an option for me.

So if anyone has some sort of miracle solution, it would be much appreciated.