If clang backend in Qt Creator stops working in Ubuntu 20.04 LTS

Ubutntu 20.04 LTS has Qt Creator  4.11.0 in APT, and if you install this and open a source file you'll encounter continuous error message of  The clangbackend process has finished unexpectedly and was restarted. If you open shell you'll see some additional messages like Error in ClangCodeModelServer::documentsClosed: Document '/path/to/sourcefile.cpp' does not exists!

For more than one year, to avoid the message, I turned off clang code model plugin from the Creator, but today I found a simple solution - downgrade clangd from 10.0(stock default install) to 8.0. What a relief!

Now I can enjoy the powerful help from libclang. :)