Welcome to Vodafone Community
From some previous posts it does appear some people are having some hiccups with their phone after updating from Marshmallow to Nougat.
It seems from what you've posted in regards to troubleshooting you've done most of what you can.
Thd only other two additional options are to clear your Phones System Cache , clear the Apps Cache and as a final resort create a Backup and Factory Reset.
I've been having issues with my banking app since updating to 7.0 and updating the App.
In the App details you should find the App developer so you can send them a bug report.
The Vodafone Team here will note what's happening and send feedback too I'm sure.
Sometimes a timely app update comes or the glitch basically clears it's self.
I updated and HTC M9 to Nougat a couple of weeks ago. All went well, but I had trouble with one particular Chromecast and a couple of apps that use location services. After about a week, all of them mysteriously settled down. My Vodafone has been fine all along, though it was reporting itself unavailable late last week (back online today).
In my case it does not reach to login process even. As soon as you open the app it say above mentioned error message. Looks like app does not connecting with vodafone system.
We've sent you a private message for another thread today.
Please add this issue to the email and we'll be happy to look into this for you.