[Xiaomi Mi 6 Themes, Apps, and Mods] geronimoge:
Thanks for your reply. Here's exactly what i did:

1. Rebooted to official TWRP 3.2.1.1.

2. Formatted data

3. wiped cache, dalvik cache, data and system

4. Installed 8.3.29 firmware.

5. Installed official LOS 15.1

6. Booted system and rebooted to TWRP

7. Installed Magisk 16.4

8. Booted system

9. Flashed C2API_enabler_v1500.zip

10. Got proof, that "persist.camera.HAL3.enabled=1" when using the command "getprop | grep camera"

10. Installed some apps, e.g. AFWall+

11. Installed this gcam version

12. Denied internet access for gcam using AFWall+

13. Went in the gcam settings and gave all permissions

14. Opened gcam and got fc

15. Went in the gcam settings, closed the app and cleared cache and data

16. Opened gcam and got fc

17. Went in the gcam settings, closed the app and cleared cache and data

18. Did a reboot to system

19. Opened gcam and got fc

I also tried the steps 13 to 19 in different order but that didn't help as well.

As i wrote earlier, i don't have GApps installed.

Besides MGC's version also this Arnova's version didn't work for me.

Log is always telling me:

[date time 5259:5259 E/AndroidRuntime]
FATAL EXCEPTION: main
Process: com.google.android.GoogleCamera, PID: 5259
java.lang.RuntimeException: Unable to get provider com.google.android.apps.camera.contentprovider.CameraContentProvider: java.lang.SecurityException: Failed to find provider com.google.android.gsf.gservices for user 0; expected to find a valid ContentProvider for this authority
at android.app.ActivityThread.installProvider(ActivityThread.java:6242)
at android.app.ActivityThread.installContentProviders(ActivityThread.java:5805)
at android.app.ActivityThread.handleBindApplication(ActivityThread.java:5722)
at android.app.ActivityThread.-wrap1(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1656)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:440)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
Caused by: java.lang.SecurityException: Failed to find provider com.google.android.gsf.gservices for user 0; expected to find a valid ContentProvider for this authority
at android.os.Parcel.readException(Parcel.java:2004)
at android.os.Parcel.readException(Parcel.java:1950)
at android.content.IContentService$Stub$Proxy.registerContentObserver(IContentService.java:768)
at android.content.ContentResolver.registerContentObserver(ContentResolver.java:1924)
at android.content.ContentResolver.registerContentObserver(ContentResolver.java:1913)
at hzy.a(Unknown Source:34)
at hzy.a(Unknown Source:6)
at bqd.a(Unknown Source:43)
at dwq.a(Unknown Source:20)
at kgq.a(Unknown Source:15)
at dxq.a(Unknown Source:7)
at kgq.a(Unknown Source:15)
at cag.a(Unknown Source:118)
at com.google.android.apps.camera.legacy.app.app.CameraApp.a(Unknown Source:187)
at com.google.android.apps.camera.legacy.app.app.CameraApp.cameraContentProviderComponent(Unknown Source:0)
at com.google.android.apps.camera.contentprovider.CameraContentProvider.onCreate(Unknown Source:48)
at android.content.ContentProvider.attachInfo(ContentProvider.java:1917)
at android.content.ContentProvider.attachInfo(ContentProvider.java:1892)
at com.google.android.apps.camera.contentprovider.CameraContentProvider.attachInfo(Unknown Source:2)
at android.app.ActivityThread.installProvider(ActivityThread.java:6239)
... 10 more