iPhone won't restore

I have recently updated the software on my iPhone 5s to iOS 8.4. I have realised that the other storage on my phone shot up to 5/6 Gb, I have tried to restore my phone to a previous back up or even factory settings to get rid of this issue. Every time I have tried to restore it I got the same error message...

I have tried this same method about 15 times and don't know how to get rid of this big 'other storage'.

Any advice?