When I click on “New Entry” in the Data Tab … I get this.
…every time. Which is a bit odd ?
It then creates a blank, I think, which I can update.
Faced similar issues, unfortunately before i can figure out a recreate procedure or investigate it seemed to correct it self. Incognito seems to help
Thanks ! Not just me then 
Isn’t stopping me doing anything, but can’t work out why it happens.
Can you file a bug report (the usual thing in these cases)?
Yes, will do. It is hard to pin down the exact circumstances though, hence the wider question.
Well you said every time, so it has to be reliably happening right?
The “modify” dialog when you click “new” is reliable.
What fields are available to update isn’t. Sometimes fields that should be entereable don’t appear.
Well if you can identify a type where that happens that’d be super helpful.
Bug on the way