Stops playing sound when screen locks

[Peter]

I tried to login several times without success, same error :-(.

Finally, I thought about the "Maximum number of simultaneous user sessions" (Edit User - Profile - Maximum number of simultaneous user sessions) setting, which was set to 1 and I had already a Jellyfin session open. Increased the "Maximum number of simultaneous user sessions" setting and I was able to login.
Now, when the screen locks or when using another app simultaneously, Finamp (mp3) keeps playing :-).

Thank you all for your suggestions, especially @theguymadmax !