Dunno what you did, but I can’t reproduce the problem.

When the Auto-Sandbox pop up kicks in and you select “Open sandboxed”, you can not tick the “remember this” option.

So I opened the prog sandboxed. Closed it again and restarted. Again, the SB-pop up asks me. This time I selected “do not open sandboxed”. Works fine.
I closed again, restarted, selected “do not open sandboxed” and ticked the “remeber this” option.
Closed and opened again: no pop up, not sandboxed.
Closed again, edited the exclusion list by deleting the prog entry, restarted the prog: voilá, pop up.

Everything is just working perfectly.

So I come to believe that you messed up something with your Avast installation, regarding your last post about this.

I would recommend to uninstall avast completely, delete all folders that Alwil / Avast might leave on the PC and then reinstall Version 6.0.1044.