I couldn't find a similar thread and this bug makes me so upset because it almost always happens.
What are the steps you took to get to the error/issue?
0. (I downloaded several playlists and I almost have the 3333 song limit but still have free space on my ext SD card)
1. disconnect Wi-Fi, deactivate streaming via cellular network
2. start spotify, play a offline playlist
3. listen to some music
4. go to Android home screen, activate WiFi, device connects to a network
5. after a few seconds, the music suddenly pauses, sometimes during a song, but most times while the next (downloaded=offline) song should start playing.
6. spotify reacts veeeery slowly, it takes several seconds (sometimes half a minute) until the spotify screen loads (until the cover is shown and the forward/play/pause keys react) when I reopen spotify. Often, the song that just played finished  and the song's progress bar stopped at the end of the song and just stays there for half a minute or so.

What I expected to happen
The music should just continue playing with high priority, no music is downloaded, no special system ressources are used (games, big apps, facebook, ...)
Did you restart your device when troubleshooting in section B?
Oh, you want me to restart my device 20 times a day? Well, I restart my device several times a week but this is a permanent error.
I always listen to spotify while studying and of course Wi-Fi is disabled. Every like 20-30 Minutes when I connect my Wi-Fi, this happens.
Did you read the connection troubleshooting tips?
Yes: It's a permanent error, in most cases I don't have access to the touter, Windows version works, no anti-virus

Your device
Galaxy S4 I-9505 (European)

Device’s Operating System
4.4.2 KitKat
What type of account do you have?
Premium

Space left on your device
692 MB

Space left on your External SD card (if applicable)
491 MB but also happend with roughly 1,2GB
Is your device rooted?
No
Are you using a custom ROM, if so which one, exactly?
No
Did you set your external SD card as storage location?
Yes, No
Did you manually install an older version before installing the latest version from the Play Store?
No
Do you have the same issue when switching between WiFi/Mobile Data (If applicable)?
Yes, problem is the same with Mobile Data (GSM)
If you said "No", when does the issue not occur?
-
My mobile Spotify version
1.4.0.631
My mobile provider and country
T-Mobile (Germany)
My username
<--
Do you have a Spotify (non-Facebook) login?
yes
Are you logging in with Facebook or with your Spotify login details?
No
Do you have any screenshots you can attach to more clearly explain your issue?
Not yet, I could make a tons of videos of spotify problems if you want to. Like just opening the app and waiting for a year.