malware - What is this lock screen with ads and how do I remove it?

I think my device got a malware. A lock screen with ads sometimes appear, and I must swipe an area at the bottom of the screen to open the phone. I am in Eastern Europe, some ads are in Slavic languages, and they are seemingly related to travel.

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I have seen ads for Lot Air (Polish carrier). I recently went to their site.

Considering my recent surfing activity and the nature of the ads, I guess the malware is connected to airlines active in Eastern Europe.

The lock screen also promotes the app "DU Quick Charger" by claiming it has recharged the phone extra fast during screen off. (!?) It also shows battery level. I have never installed DU Quick Charger. I doubt DU Quick Charger is installed on the phone.

The problem started after installing Ryanair app. The problem is maybe, but not surely, related to this. Uninstalling Ryanair and restarting the phone doesn't remove the occasional appearance of this lock screen. After a while the situation has changed for the worse, now this lock screen appears after EVERY screen off.

I have always used Avast. I am not rooted and only install via Google Play.

What is this? And how do I remove it?

Answer

This lockscreen replacement is caused by an ad from DU Quick Charge

I am pretty sure that this lockscreen was added to the app "ES App Locker". Many User-Revievs are stating that this app comes with lots of unwanted ads and lockscreen-replacements.
See this review:

This is what they are answering to most people:

So you either have the option to turn this settings off apparently or remove this application.

You can check the user-reviews here: Google Play Story

Disabling DU Quick Charge may not work on devices. In this case insure that you completely remove the application. If the quick charge screen continues to display, check to see if you have any other apps that have this "adware" and remove them.

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EDIT:

Based on the information seen in another question I wanted to update this answer. It seems like quite a handful of applications have added this lockscreen replacement in their latest updates. There is one Reddit discussion which mentions a few apps.

As @Andrew T. pointed out - the main cause of this lockscreen replacement is the advertisement of DU Quick Charge - developed by DU APPS STUDIO.

If you are experiencing similar problems then check this helpful answer by @Firelord to check which app might cause this.

_{I'll try to update this answer from time to time.}