Failed to load system tools

[servicesigu]

Hello,

I have created 2 new pyhton toolboxes in the directory "C:\Program Files\ArcGIS\Pro\Resources\ArcToolBox\Toolboxes"

called empty2.pyt and empty1.pyt  with no tool (code below)

When opening ArcGis Pro 2.4.2, this message comes:

The toolboxes are correctly visibles.

I also tried by copying 2 times existing python toolboxes (like Space Time Pattern Mining Tools.pyt), the same message comes.

In fact it seems that if more than one python toolbox is copied to this directory, this message appears.

If only one is present no message.

How to correct this error ?

I need new toolboxes to be enabled for all projects (default toolboxes), like ESRI toolboxes.

Thanks in advance for your replies.