Hello again and thanks for your answers!
Something magical happened - since I created this account to share my problem here yesterday, I can use EN again and the warnings are gone. I don't know exactly why, maybe it has something to do with me logging in again (on the website)? I don't have to log in / log out on my pre-installed EN App on my PC, but that's the only explanation I could think of...
Anyways, I have followed @gazumped advice and checked how many devices are associated with my account. It turned out to be 3 accounts: my PC, my phone (where I thought I deleted EN) and my very old phone which I haven't used for 3 years now. That could explain a lot, thank you!
@PinkElephant Good to know. Luckily, that's not the case! 😅
@agsteeleThanks, I will try that!
Thanks again for your help.