Hello @npmagalhaes ,
Thank you for the follow up on this issue. Your .addin file FullClassName tag seems fine to me.
I had encountered the same error you are facing just a few days ago, I went through this article for troubleshooting my code. Maybe you could have a look first and see if you could get a solution.
Troubleshooting .addin Revit File
It seems your .addin file is referencing the wrong assembly .dll file probably from some other projects within your computer you had recently build before. You also, close up all the visual studio projects and re-open the only project you are dealing with, perform a fresh build and launch your addin.
Also, make sure you:
Note: You should always set the Copy Local property of RevitAPI.dll to false for new projects. This saves disk space, and prevents the Visual Studio debugger from getting confused about which copy of the DLL to use. Right-click the RevitAPI.dll, select Properties, and change the Copy Local setting from true (the default) to false.
If the issue persists kindly share minimum reproducible case of your code and .addin file.
Carol Gitonga,

Developer Advocacy and Support,
ADN Open