I've had the same "Assertion failed" and since I was not able to find any solution, I thought posting mine - maybe it helps someone.
What caused the error: I was running low on disk space on the system partition, and I suppose that when I closed chief architech some files were corrupted.
What I've tried:
-first I tried the "repair" option , restarted, same issue
-uninstalled and manually removed all registry references to the program (because just uninstalling leaves a lot of debris behind) ; restart laptop / then re-install; still no luck
-tried to search over internet for a solution with no clear or final answer;
-did a backup of my "Chief Architect Premier X6 Data" from c:\Users\your_user_folder_name\Documents\ ; take special care to backup your "Archives" sub-folder
-removed the whole "Chief Architect Premier X6 Data" and started again the program with no error this time.
-recovered the subfolders in "Chief Architect Premier X6 Data" , one by one, and restarted each time the program to see what was causing the error
In the end I found out that the file "c:\Users\your_folder\Documents\Chief Architect Premier X6 Data\Toolbars\libraryItems.tbdata" was the corrupted one.
You don't have to follow all my steps, and can safely remove the mentioned file (you can create back it up before if you really want) as it will be auto-re-created when you close the program.
So the short solution should be: remove "c:\Users\your_folder\Documents\Chief Architect Premier X6 Data\Toolbars\libraryItems.tbdata" file, and restart chief architect.