Thinking myself extremely clever, I attempted to add - via DOpus.aliases.Add() - the quickest alias possible, simply named "/". I thought I could just hit two forward slashes in a row... Didn't work, DOpus crashed.
Subsequently it crashed every time I even dared type a slash. I found that in folderaliases.oxc a new path node had been added without a label attribute. After removing this and restarting DOpus, everything is fine again.
The solution to this is obvious: Don't try stupid things! 
But I thought I'd report it anyway...