Problem
Please check that you use THE SAME Python interpreter for IDE and CLI. If you are on Windows, remove multiple Pythons from a system and use the latest Python 3.7. See instruction Redirecting...
So I found the file I needed to modify.
I modified the line 144 of the file .platformio/penv/lib/python3.7/site-packages/platformio/builder/main.py
And I replaced:
env.SConsignFile( join("$BUILD_DIR", ".sconsign.py%d%d" % (sys.version_info[0], sys.version_info[1])) )
by
env.SConsignFile( join("$BUILD_DIR", ".sconsign.dblite") )
And now it’s working.
I think this is a bug because the mbed framework contains many many files.