I don't see any error in you post. Might be a quite special root cause and I would recommend to log a support case at https://gateway.sdl.com as it will require to send files and projects.

the error I get is the one in the title: "main assembly for source satellite assembly not found". I don't have support contract so I guess I cannot open a case there, right?

The method on how WinForm satellite assemblies are localized has been changed. It will be necessary for SDL Passolo to also access the main assembly in order to extract information important for the localization process like visual inheritance and other. So you also need the main assembly to fix the error.

Hey Roberto,
I get this all the time when I am working on projects that I created in 2018+.NET when I am trying to use a scan a target file from another folder than the Project or Source path.
Especially if the satellite might be a different version than the main assembly. The only way to correct that issue is to put the satellite assembly in the folder of the Target Files and then Scan the Target. You must have the main assembly in the source folder no matter the version.
Make sure also that when you are working on Satellites that you have updated the files with the latest version of the Main Assembly. It is a rare occurence, but sometimes either the main assembly will be a newer version but not show up as needing to be updated, or the main assembly will have been updated, but the satellites don't list that they need to be updated.
In this case, I do an update of the main (source) and then do an update of all of the target strings. Once the system knows they are all in sync, then it will work.