I face the same issue - resort kill and try again 2-3 times.
Connecting the phone with USB debugging enabled shows this:
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Local Time: 03/25/2021 21:48:40 +00:00
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Device Serial Number: 9E3969FB-4681-464B-B5FF-BC0DB01126C6
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Roon Version: 1.8 (build 778) stable
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 OS Version: Android 10
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Mono Version: 6.4.0 ((no/fe64a476)
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Application Domain: RoonMobile.exe
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Assembly Codebase: file:///RoonMobile.exe
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Assembly Full Name: RoonMobile, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Exception Source: BrooEngine
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Exception Type: System.NullReferenceException
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Exception Target Site: LoadContext.GetIcon
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Exception Message: Object reference not set to an instance of an object
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Exception Data: none
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 --[ Stack Trace ]------------
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.LoadContext.GetIcon(String name)
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 58, N 156
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.BaseImageDrawable.LoadImage()
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 119, N 419
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.BaseImageDrawable.CreateNode(Widget w)
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 0, N 27
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.AnimatedDrawable/Node…ctor(AnimatedDrawable spec, Widget w)
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 86, N 523
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.AnimatedDrawable.CreateNode(Widget w)
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 0, N 51
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.DrawNode.CreateHelper(Widget w, Drawable spec, WidgetState state, DrawNode old, RenNode parent)
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 15, N 91
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.ImageWidget._Update()
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 47, N 195
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.Widget.Update()
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 227, N 712
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.Engine.Update()
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 157, N 383
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.DeviceTarget.GuiMainLoop()
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 341, N 919
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Broo.Engine.Engine.GuiMainLoop()
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 BrooEngine, IL 0, N 31
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Sooloos.Application.RoonMain(String[] args)
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 RoonMobile.exe, IL 673, N 2403
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 Sooloos.Application.Main(String[] args)
2021-03-25 22:48:40.685 16550-16631/? E/Roon: 03/25 21:48:40 RoonMobile.exe, IL 0, N 19
2021-03-25 22:49:09.882 16550-16550/? D/Roon: Pausing
I am happy to share details on setup, the TL;DR is
Rock on NUC
Roon Bridge on PI 4b
Client on Huawei Mate 20 - Android 10
Rock and Bridge connected via Ethernet - client via WI-FI all on same TP Link Router
The problem arose after 1.8 upgrade
I appreciate that not all problems are easily fixed ASAP, but there seems to be a lack of feedback from @support on this issue