Hi Marina,
Not sure why you can't recreate it. I just deleted all the data as you suggested and it is still doing the same. I have shared the app with a href="mailto:suport@appery.io" rel="nofollow"suport@appery.io/a if you test it you'll see it fails every time after adding a new client. Then if you change any of the values in the client collection like the order column it suddenly works when viewing the clientList page.
Very strange.