ios build Failed tundra.dag no longer valid. File Signature timestamp changed: il2cpp_settings.json

I am having problems with iOS build again.
I tried my Mouse4Cat app build using original source and it built ok in 2018 and 2019 versions but failed in 2020 and it looks like I had something basically wrong so started again setting a base on 2020.
I completely rebuilt the app from scratch only copying in the picture files.
I built and tested on PC platform ok so changed to iOS and verified I had the necessary items on develloper.apple.com matched to the settings and so tried a build and got the above error buried in the console output which I have copied below.
Is there anyone who could help me solve this with a hint as to what I have missed.

----
Exception: Failed running D:\Program Files\2020.3.12f1\Editor\Data\il2cpp/build/BeeSettings/offline/bee.exe --useprebuiltbuildprogram --no-colors
stdout:
[ ] Require frontend run. tundra.dag no longer valid. FileSignature timestamp changed: il2cpp_settings.json
[ 0s] Executed build program. Created build graph with 4 nodes.
stderr:
UnityEditorInternal.Runner.RunProgram (UnityEditor.Utils.Program p, System.String exe, System.String args, System.String workingDirectory, UnityEditor.Scripting.Compilers.CompilerOutputParserBase parser) (at <dc14d30a5e0540ee9311ed6df63d9074>:0)
UnityEditorInternal.Runner.RunManagedProgram (System.String exe, System.String args, System.String workingDirectory, UnityEditor.Scripting.Compilers.CompilerOutputParserBase parser, System.Action`1[T] setupStartInfo) (at <dc14d30a5e0540ee9311ed6df63d9074>:0)
UnityEditorInternal.IL2CPPBuilder.RunIl2CppWithArguments (System.Collections.Generic.List`1[T] arguments, System.Action`1[T] setupStartInfo) (at <dc14d30a5e0540ee9311ed6df63d9074>:0)
UnityEditorInternal.IL2CPPBuilder.ConvertPlayerDlltoCpp (UnityEditor.Il2Cpp.Il2CppBuildPipelineData data) (at <dc14d30a5e0540ee9311ed6df63d9074>:0)
UnityEditorInternal.IL2CPPBuilder.Run () (at <dc14d30a5e0540ee9311ed6df63d9074>:0)
UnityEditorInternal.IL2CPPUtils.RunIl2Cpp (System.String tempFolder, System.String stagingAreaData, UnityEditorInternal.IIl2CppPlatformProvider platformProvider, System.Action`1[T] modifyOutputBeforeCompile, UnityEditor.RuntimeClassRegistry runtimeClassRegistry) (at <dc14d30a5e0540ee9311ed6df63d9074>:0)
UnityEditor.iOS.PostProcessiPhonePlayer.CrossCompileManagedDlls (UnityEditor.iOS.PostProcessiPhonePlayer+BuildSettings bs, UnityEditor.iOS.PostProcessiPhonePlayer+ProjectPaths paths, UnityEditor.AssemblyReferenceChecker checker, UnityEditor.RuntimeClassRegistry usedClassRegistry, UnityEditor.Build.Reporting.BuildReport buildReport) (at C:/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/BuildPostProcessor.cs:908)
UnityEditor.iOS.PostProcessiPhonePlayer.PostProcess (UnityEditor.iOS.PostProcessiPhonePlayer+BuildSettings bs, UnityEditor.iOS.PostProcessiPhonePlayer+ProjectPaths paths, UnityEditor.RuntimeClassRegistry usedClassRegistry, UnityEditor.Build.Reporting.BuildReport buildReport) (at C:/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/BuildPostProcessor.cs:718)
UnityEditor.iOS.PostProcessiPhonePlayer.PostProcess (UnityEditor.iOS.PostProcessorSettings postProcessorSettings, UnityEditor.Modules.BuildPostProcessArgs args) (at C:/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/BuildPostProcessor.cs:663)
UnityEditor.iOS.iOSBuildPostprocessor.PostProcess (UnityEditor.Modules.BuildPostProcessArgs args) (at C:/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/ExtensionModule.cs:45)
Rethrow as BuildFailedException: Exception of type 'UnityEditor.Build.BuildFailedException' was thrown.
UnityEditor.iOS.iOSBuildPostprocessor.PostProcess (UnityEditor.Modules.BuildPostProcessArgs args) (at C:/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/ExtensionModule.cs:49)
UnityEditor.Modules.DefaultBuildPostprocessor.PostProcess (UnityEditor.Modules.BuildPostProcessArgs args, UnityEditor.BuildProperties& outProperties) (at <dc14d30a5e0540ee9311ed6df63d9074>:0)
UnityEditor.PostprocessBuildPlayer.Postprocess (UnityEditor.BuildTargetGroup targetGroup, UnityEditor.BuildTarget target, System.String installPath, System.String companyName, System.String productName, System.Int32 width, System.Int32 height, UnityEditor.BuildOptions options, UnityEditor.RuntimeClassRegistry usedClassRegistry, UnityEditor.Build.Reporting.BuildReport report) (at <dc14d30a5e0540ee9311ed6df63d9074>:0)
UnityEngine.GUIUtilityrocessEvent(Int32, IntPtr, Boolean&)
Build completed with a result of 'Failed' in 73 seconds (73376 ms)
UnityEngine.GUIUtilityrocessEvent (int,intptr,bool&)
UnityEditor.BuildPlayerWindow+BuildMethodException: 2 errors
at UnityEditor.BuildPlayerWindow+DefaultBuildMethods.BuildPlayer (UnityEditor.BuildPlayerOptions options) [0x002ca] in <dc14d30a5e0540ee9311ed6df63d9074>:0
at UnityEditor.BuildPlayerWindow.CallBuildMethods (System.Boolean askForBuildLocation, UnityEditor.BuildOptions defaultBuildOptions) [0x00080] in <dc14d30a5e0540ee9311ed6df63d9074>:0
UnityEngine.GUIUtilityrocessEvent (int,intptr,bool&)

----