@Pasik2: thanks A LOT for the link. Not sure why i could not find it. Now it was possible to check what app is using all the RAM. It was the android layer. It uses about 1,5 GB.
Is it possible that the “shift + M” function is not working in top, because i deinstalled busybox?
Again, thanks to everyone for the infos. It helped a lot to find out what the problems are. I will try to only start the android layer when i play games now. It is just weird that i could listen to music in the browser AND play i.e. world of tanks at the same time on the Jolla C. This is not possible anymore. Well, maybe a patch will make the RAM usage go back to normal one day.
edit: i also found out that the browser is using about 1,5 GB of RAM or even more with only two tabs opened.
edit2: btw: what also helped a lot now was to change the out of memory killer app to 85-90% with:
echo 85 > /proc/sys/vm/overcommit_ratio
i found this on [Discussion] Oom Killer fine tuning (overcommit_ratio value) - together.jolla.com
One bug is still very annoying. I hope you guys can help with that too. Why is only the 5 Ghz wifi band working? It is even worse than any wifi i ever had on any device. Any idea or anyone who got both bands working?