It seems like it's a problem with Chrome. I also ran through the troubleshooting points that Squarespace sent me https://support.squarespace.com/hc/articles/115004581087 but didn't seem to help.
I could add a subscriber if i log in via Safari rather than chrome.
having reported that back to squarespace they replied: After further review, we're currently investigating an issue with not being able to manually add and import contacts. We’ve noted your report with our Engineering and Design teams for review. They’ll use this feedback when creating fixes and improvements for future releases.
There are many variables that we need to test against before we can make changes, and we can’t guarantee changes for this issue in the immediate future. That said, our teams consider all feedback, and your input is always encouraged.
For now, occasionally, we've seen from other users that using a different browser can resolve the issue, but this doesn’t consistently resolve the behavior.
So, I guess it's an issue they know about, which may or may not get fixed at some point....?