Hi! Unfortunately, I got the exact same problem here. When I click on the "Restore Purchases" Button, it just doesn't react.
I'm logged in with the same Apple ID I got the Pro Version license with.
Really a bummer, because it worked so well in the past and I recommended the app to so many friends.
Hope you will find a solution!
Diarium version: 2.7.13 (on Mac) 2.7.12 (on iPhone)
Device: MacBook Pro M1 Late 2020
OS version: macOS Big Sur 11.4