Importing new contacts to mailing lists

I would like to find out if anyone else is having the same issue as me:

I am frequently having issues with importing contacts in 2 different browsers (Edge and Chrome). I've added people as individuals and as a CSV (the format is correct: email, first name, surname). I've cleared the cache and refreshed pages. Basically I've been doing all the things that Squarespace Support tell me to do, and for a while it works.... and then it doesn't. It keeps telling me it failed.

Does anyone else have the same issue? How do you get round this?

Squarespace tells me things that only temporarily help. But then I have the same problem all over again, and it is really frustrating! I never use to have this issue before on Edge.