Hello! I've heard of a few ways to fix this, but one I know works for sure is to go into your inbox and create a new conversation category. On desktop it's to the left of your conversations. On mobile you need to hit the "categories" button under "Start conversation". As soon as you successfully create a new category, Iwaku will be able to find and list the default category of "All Conversations", as well as the new one you just created, in the formerly blank drop down menu.
I believe the newly added category can be deleted after that if you don't want to use it, but I haven't tested it myself, so if you decide to do that please report back and let me know. ^_^