Error installing: Cannot use an old toolchain (missing ToolchainVersion.txt file)

When trying to compile the new hotfix 4.23.1 the editor on Ubuntu 16.04 from the current release branch on GitHub (tagged as 4.23.1-release) I get the following result:

$ (release):/opt/UnrealEngine> ./Setup.sh 
Registering git hooks... (this will override existing ones!)
Fixing inconsistent case in filenames.
Setting up Mono
Checking dependencies...
The following file(s) have been modified:
  Engine/Binaries/ThirdParty/Mono/Linux/etc/mono/registry/last-btime
Would you like to overwrite your changes (y/n)? y
Updating dependencies: 100% (1/1), 0.6 MiB cached, done.
Result: 0
Register the engine installation...
Installing a bundled clang toolchain
Toolchain already installed skipping.

Target arch set to: x86_64-unknown-linux-gnu
Building ThirdParty libraries

If you don't see SUCCESS message in the end, then building did not finish properly.
In that case, take a look into /opt/UnrealEngine/Engine/Build/BatchFiles/Linux/BuildThirdParty.log for details.

No third party libs needed to be built locally

**********  SUCCESS ****************
Setup successful.
$ (release):/opt/UnrealEngine> ./GenerateProjectFiles.sh 
Attempting to set up UE4 pretty printers for gdb (existing UE4Printers.py, if any, will be overwritten)...
	updated UE4Printers.py
	found necessary entries in ~/.gdbinit file, not changing it.

Setting up Unreal Engine 4 project files...

Fixing inconsistent case in filenames.
Setting up Mono
Generating data for project indexing... 100%
Writing project files... 100%
$ (release):/opt/UnrealEngine> make
make[2]: *** No rule to make target 'CMakeFiles/FakeTarget.dir/build'.  Stop.
CMakeFiles/Makefile2:1507: recipe for target 'CMakeFiles/FakeTarget.dir/all' failed
make[1]: *** [CMakeFiles/FakeTarget.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
$ (release):/opt/UnrealEngine> make all
make[2]: *** No rule to make target 'CMakeFiles/FakeTarget.dir/build'.  Stop.
CMakeFiles/Makefile2:1507: recipe for target 'CMakeFiles/FakeTarget.dir/all' failed
make[1]: *** [CMakeFiles/FakeTarget.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
$ (release):/opt/UnrealEngine> make CrashReportClient ShaderCompileWorker UnrealLightmass UnrealPak UE4Editor
Fixing inconsistent case in filenames.
Setting up Mono
Building CrashReportClient...
Using 'git status' to determine working set for adaptive non-unity build (/opt/UnrealEngine).
Creating makefile for CrashReportClient (no existing makefile)
------- Build details --------
Using toolchain located at '/opt/UnrealEngine/Engine/Extras/ThirdPartyNotUE/SDKs/HostLinux/Linux_x64/v15_clang-8.0.1-centos7/x86_64-unknown-linux-gnu'.
Using clang (/opt/UnrealEngine/Engine/Extras/ThirdPartyNotUE/SDKs/HostLinux/Linux_x64/v15_clang-8.0.1-centos7/x86_64-unknown-linux-gnu/bin/clang++) version '8.0.1' (string), 8 (major), 0 (minor), 1 (patch)
Using bundled libc++ standard C++ library.
Using lld linker
Using llvm-ar : /opt/UnrealEngine/Engine/Extras/ThirdPartyNotUE/SDKs/HostLinux/Linux_x64/v15_clang-8.0.1-centos7/x86_64-unknown-linux-gnu/bin/llvm-ar
Using fast way to relink  circularly dependent libraries (no FixDeps).
------------------------------
ERROR: Cannot use an old toolchain (missing ToolchainVersion.txt file, assuming version earlier than v11)
CMakeFiles/CrashReportClient.dir/build.make:57: recipe for target 'CMakeFiles/CrashReportClient' failed
make[3]: *** [CMakeFiles/CrashReportClient] Error 5
CMakeFiles/Makefile2:4168: recipe for target 'CMakeFiles/CrashReportClient.dir/all' failed
make[2]: *** [CMakeFiles/CrashReportClient.dir/all] Error 2
CMakeFiles/Makefile2:4175: recipe for target 'CMakeFiles/CrashReportClient.dir/rule' failed
make[1]: *** [CMakeFiles/CrashReportClient.dir/rule] Error 2
Makefile:1782: recipe for target 'CrashReportClient' failed
make: *** [CrashReportClient] Error 2

The concrete problem is logged as ERROR: Cannot use an old toolchain (missing ToolchainVersion.txt file, assuming version earlier than v11). I haven’t encountered this problem when building versions 4.22 and 4.23 and I can’t seem to find what was changed in between to cause this. Has anyone got a clue?

PS: What’s the deal with this FakeTarget that doesn’t have a rule when building with the default make command? It didn’t work on either version I tried. As a workaround I had to build the targets manually as described on the official Linux native build site (also shown in the logs above).

{ b u m p }

Please manually delete Engine/Extras/ThirdPartyNotUE/SDKs/HostLinux/Linux_x64 and then re-run Setup.sh. I.e. execute

rm -rf Engine/Extras/ThirdPartyNotUE/SDKs/HostLinux/Linux_x64
./Setup.sh

from the root engine directory.

It worked magnificently! Thank you!