Okay, I will try switching to the default Layout.
I tried running the project in 2020, but was drowned in error messages and gave up immediately. I will get back in a couple of minutes with an edit of this post concerning the Layout change. Thank you for your swift reply!
Weird thing is, I never really changed much concerning the Layout. Only switched to Wide screen preset.
Just now when restoring to factory, I got the error
Failed to destroy editor windows: #1
FallbackEditorWindow
UnityEditor.<>c:<ReloadWindowLayoutMenu>b__28_0() (at /home/bokken/buildslave/unity/build/Editor/Mono/GUI/WindowLayout.cs:224)
EDIT:
ArgumentException: Getting control 5's position in a group with only 5 controls when doing repaint
Aborting
UnityEngine.GUILayoutGroup.GetNext () (at /home/bokken/buildslave/unity/build/Modules/IMGUI/LayoutGroup.cs:122)
UnityEngine.GUILayoutUtility.DoGetRect (System.Single minWidth, System.Single maxWidth, System.Single minHeight, System.Single maxHeight, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[ ] options) (at /home/bokken/buildslave/unity/build/Modules/IMGUI/GUILayoutUtility.cs:464)
UnityEngine.GUILayoutUtility.GetRect (System.Single minWidth, System.Single maxWidth, System.Single minHeight, System.Single maxHeight, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[ ] options) (at /home/bokken/buildslave/unity/build/Modules/IMGUI/GUILayoutUtility.cs:453)
UnityEditor.EditorGUILayout.GetToggleRect (System.Boolean hasLabel, UnityEngine.GUILayoutOption[ ] options) (at /home/bokken/buildslave/unity/build/Editor/Mono/EditorGUI.cs:9710)
UnityEditor.EditorGUILayout.Toggle (UnityEngine.GUIContent label, System.Boolean value, UnityEngine.GUILayoutOption[ ] options) (at /home/bokken/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7811)
DesktopStandaloneBuildWindowExtension.ShowPlatformBuildOptions () (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/DesktopStandaloneBuildWindowExtension.cs:186)
UnityEditor.BuildPlayerWindow.ShowBuildTargetSettings () (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPlayerWindow.cs:698)
UnityEditor.BuildPlayerWindow.OnGUI () (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPlayerWindow.cs:386)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[ ] parameters, System.Globalization.CultureInfo culture) (at <9577ac7a62ef43179789031239ba8798>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[ ] parameters, System.Globalization.CultureInfo culture) (at <9577ac7a62ef43179789031239ba8798>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[ ] parameters) (at <9577ac7a62ef43179789031239ba8798>:0)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at /home/bokken/buildslave/unity/build/Editor/Mono/HostView.cs:381)
UnityEditor.HostView.Invoke (System.String methodName) (at /home/bokken/buildslave/unity/build/Editor/Mono/HostView.cs:375)
UnityEditor.HostView.OldOnGUI () (at /home/bokken/buildslave/unity/build/Editor/Mono/HostView.cs:151)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Action onGUIHandler, System.Boolean canAffectFocus) (at /home/bokken/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:293)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Action onGUIHandler, System.Boolean canAffectFocus) (at /home/bokken/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:508)
UnityEngine.UIElements.IMGUIContainer.DoIMGUIRepaint () (at /home/bokken/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:463)
UnityEngine.UIElements.UIR.RenderChainCommand.ExecuteNonDrawMesh (UnityEngine.UIElements.UIR.DrawParams drawParams, System.Boolean straightY, System.Single pixelsPerPoint, System.Exception& immediateException) (at /home/bokken/buildslave/unity/build/Modules/UIElements/Renderer/UIRenderer/UIRenderers.cs:107)
Rethrow as ImmediateModeException
UnityEngine.UIElements.UIR.RenderChain.Render (UnityEngine.Rect viewport, UnityEngine.Matrix4x4 projection, UnityEngine.UIElements.PanelClearFlags clearFlags) (at /home/bokken/buildslave/unity/build/Modules/UIElements/Renderer/UIRChainBuilder.cs:392)
UnityEngine.UIElements.UIRRepaintUpdater.DrawChain (UnityEngine.Rect viewport, UnityEngine.Matrix4x4 projection) (at /home/bokken/buildslave/unity/build/Modules/UIElements/Renderer/UIRRepaintUpdater.cs:68)
UnityEngine.UIElements.UIRRepaintUpdater.Update () (at /home/bokken/buildslave/unity/build/Modules/UIElements/Renderer/UIRRepaintUpdater.cs:57)
UnityEngine.UIElements.VisualTreeUpdater.UpdateVisualTreePhase (UnityEngine.UIElements.VisualTreeUpdatePhase phase) (at /home/bokken/buildslave/unity/build/Modules/UIElements/VisualTreeUpdater.cs:84)
UnityEngine.UIElements.Panel.UpdateForRepaint () (at /home/bokken/buildslave/unity/build/Modules/UIElements/Panel.cs:721)
UnityEngine.UIElements.Panel.Repaint (UnityEngine.Event e) (at /home/bokken/buildslave/unity/build/Modules/UIElements/Panel.cs:747)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at /home/bokken/buildslave/unity/build/Modules/UIElements/UIElementsUtility.bindings.cs:387)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at /home/bokken/buildslave/unity/build/Modules/UIElements/UIElementsUtility.bindings.cs:194)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at /home/bokken/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:197)
Running Unity3D 2019 with target platform Windows in Ubuntu now returns the above error AFTER opening the Build Settings in the Editor.
Layout is Restored to Factory Settings and no initial Error after Start-up.
Only Build Setting for Target Platform within the Editor is Linux again.
Starting the Editor from the Hub with Linux as target platform returns no Errors.