Heya,
This is indeed bugged at the moment, as listed as such in the current known issues on the Support page and in our Sticky here.
Bug: Unable to open (Irradiated) Box of Assorted Parts
The event reward boxes that are mailed out to you if you have a full inventory, cannot be opened. This is being worked on, but for the time being please make sure to have space in your inventory to receive the rewards directly.
Have a few seconds to spare? Let me know how I’m doing!