[Magisk] CHV:
Solved

Additional Info to #1841:

What's wrong here and how can I fix it? This happens after a reboot of my phone, when I try to launch Youtube Vanced.

The log says it can't find /system/app/YouTube/Youtube.apk. But it's really there.

I/ActivityManager( 915): START u0 {act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000 cmp=com.google.android.youtube/.app.honeycomb.Shell$HomeActivity (has extras)} from uid 10139 pid 3674
E/ANDR-IOP( 743): IOP HAL: Received pkg_name = com.google.android.youtube pid = -1
E/OnePlusSmartBoostManager( 915): cmd: iop_start -1 com.google.android.youtube /system/app/YouTube false length = 65
E/ANDR-RAMBOOST_SERVER( 950): ramboost cmd_len = 65 cmd: iop_start -1 com.google.android.youtube /system/app/YouTube false
D/ActivityTrigger( 915): activityStartTrigger: Activity is Triggerred in full screen ApplicationInfo{f8b05ff com.google.android.youtube}
E/ActivityTrigger( 915): activityStartTrigger: not whiteListedcom.google.android.youtube/com.google.android.youtube.app.honeycomb.Shell$HomeActivity/1421542300
D/ActivityTrigger( 915): activityResumeTrigger: The activity in ApplicationInfo{f8b05ff com.google.android.youtube} is now in focus and seems to be in full-screen mode
E/ActivityTrigger( 915): activityResumeTrigger: not whiteListedcom.google.android.youtube/com.google.android.youtube.app.honeycomb.Shell$HomeActivity/1421542300
D/ActivityTrigger( 915): activityResumeTrigger: The activity in ApplicationInfo{f8b05ff com.google.android.youtube} is now in focus and seems to be in full-screen mode
E/ActivityTrigger( 915): activityResumeTrigger: not whiteListedcom.google.android.youtube/com.google.android.youtube.app.honeycomb.Shell$HomeActivity/1421542300
D/RestartProcessManager( 915): Update Total Launch Times :com.google.android.youtube
D/RestartProcessManager( 915): sLastRunningPackage (null) : com.google.android.youtube
I/ActivityManager( 915): Start proc 20507:com.google.android.youtube/u0a83 for activity com.google.android.youtube/.app.honeycomb.Shell$HomeActivity
I/Magisk ( 634): proc_monitor: [com.google.android.youtube] PID=[20507] UID=[10083]
D/OnePlusSmartBoostManager( 915): writeHotCount com.google.android.youtube, pid=20507
W/System (20507): ClassLoader referenced unknown path: /system/app/YouTube/Youtube.apk
W/linker (20507): Warning: unable to resolve "/system/app/YouTube/Youtube.apk": No such file or directory (ignoring)
W/linker (20507): Warning: unable to resolve "/system/app/YouTube/Youtube.apk": No such file or directory (ignoring)
W/ziparchive(20507): Unable to open '/system/app/YouTube/Youtube.apk': No such file or directory
E/android.youtub(20507): Failed to open APK '/system/app/YouTube/Youtube.apk' I/O error
E/ResourcesManager(20507): failed to add asset path /system/app/YouTube/Youtube.apk
E/AndroidRuntime(20311): Process: com.google.android.youtube, PID: 20311
E/AndroidRuntime(20507): Process: com.google.android.youtube, PID: 20507
W/ActivityManager( 915): Force finishing activity com.google.android.youtube/.app.honeycomb.Shell$HomeActivity
I/ActivityManager( 915): Showing crash dialog for package com.google.android.youtube u0
W/ActivityManager( 915): Activity pause timeout for ActivityRecord{1be59a0 u0 com.google.android.youtube/.app.honeycomb.Shell$HomeActivity t77333 f}
W/ActivityManager( 915): Force finishing activity com.google.android.youtube/.app.honeycomb.Shell$HomeActivity
W/SurfaceFlinger( 766): Attempting to destroy on removed layer: AppWindowToken{419191e token=Token{f1b1959 ActivityRecord{1be59a0 u0 com.google.android.youtube/.app.honeycomb.Shell$HomeActivity t77333}}}#0
I/ActivityManager( 915): Killing 20507:com.google.android.youtube/u0a83 (adj 900): crash
W/SurfaceFlinger( 766): Attempting to destroy on removed layer: 4c708c7 Application Error: com.google.android.youtube#0

Edit: Solved this. You must NOT include Youtube Vanced in the "Magisk Hide" option.