Do you get the Dialog Box "Drag & Drop failed"
I have found there is the strong possiblity of a crash very soon after this.
Sorry have no answer for you but would also love to know why this occurs and why it has not been fixed, as, in my experience, this issue/bug has been around for many versions.
I would suggest a look thru current SPR's if you can figure out how to get into that particular part of website.
Deepak Gupta seems to have a handle on searching this area and coming up with an answer.
Jon, I have the same problem and I use the same "workaround". It seems as though the mate references are not available if the toolbox part file is not opened. Once you open the bolt for example, and keep it open in the background, SW can access its mate references and it works as normal.
But lets say I right click on the part and change the configuration of the bolt- SW loses the mate references again. I have to go back to the bolt (already open in the background) and switch to the new configuration, then go back to my assembly and the mate references will work.
We recently upgraded to SW2017 SP5. Oddly enough we upgraded to fix toolbox issues, some were fixed but this one was introduced.
You are a braver man than I using toolbox.
It gives me nothing but grief