With the help from Esri support (see case #02696389) we figured out that the problem was caused by a corrupted installation of the "pandas 1.1.3" package. Today, in another attempt to upgrade the untools, ArcGIS Pro installed "pandas 1.1.5" sucessfully and after that also "untools 2.6.2". Right now, I am applying the asset package of the Communication Data Management and it seems to be running successfully as well.
This is what the messages section reports:
ArcGIS Pro 18.104.22.16883
Executing from ArcGIS Pro, 0 map(s), activeMap = False
Parallel processing enabled: 3
Asset Package schema version: 2.6
The Update Packages list only shows untools 2.6.0, even after refreshing. It is the same version of the untools that is already installed in my environment. Nevertheless, I hit the "Update All" button, but the result was the same. Which version of untools are we looking for?