iPhone 7 goes into loop when trying to charge/backup via MacBookAir

When I plug my iPhone 7 into my 2015 MacBookAir running Mojave in order to back it up, iTunes opens up and then asks whether I want to backup and I get the 'Do you want to trust this computer" message on my phone. The message disappears before I can press Trust and then goes into a repetitive binging cycle of asking me if I trust the computer (which disappears before I can press Trust) and iTunes repeatedly showing the same message.

I can charge my phone Ok via a wall charger.

Any help appreciated