Hi,
Problem is solved (in the meantime... hope it won't come back).
It appears that following the upgrade instructions is not enough...
I perform all the steps required for upgrading the project as explained in Syncfusion help.
Today I've decided to make a full textual search for old Syncfusion version references in the project.
I saw that some of the references appear in the Visual Studio as updated but the project file keeps the old references (to version 6.403, which doesn't exist on my PC).
After I cleaned all those project files manually and added the correct references the toolbox started working OK.