iOS Question Trouble compiling in Debug mode on iPhone 5

I have 2 iPhones:

iPhone 5 running iOS 10.3.4 (been using forever)
iPhone 7 running iOS 13.5.1 (just acquired)

I have just had to go thru the annual rebuild of my iOS Distribution Certificate and rebuilt it and also the iOS Development Certificate.

I added the new iPhone 7 to my Apple Developer account devices list and rebuilt the associated Development and App Store profiles.

I copied all the new certificates and profiles to the B4i Keys folder.

On the iPhone 5 I deleted the old B4i-Bridge app and installed a new one.

On the iPhone 7 I installed the B4i-Bridge app.

Using the iPhone 7 I installed my production app in both debug and release modes - everything worked fine.

Using the iPhone 5 I installed my production app in release mode - works fine.

Using the iPhone 5 I tried to install my production app in debug mode - fails with "Unable to Download App 'appname' could not be downloaded at this time [Done] [Retry]"

I assumed I had some how stuffed the UDID for the iPhone 5 in the devices list in my Apple Developer account.

I don't know how I could have done this because all I did was add the iPhone 7 device to the devices list.

Any rate I extracted the iPhone 5's UDID and compared it with that in my Apple Developer account devices list - they are identical.

So I'm stumped - the only thing I can think of is something no longer supports iOS 10.3.4

Any help appreciated...