Fields pre-filled with other user’s details

I’m running 4.7.2 on ubuntu on AWS. I’ve installed woocommerce and quite a few plugins.

When I go to updates plugins the popup that lets me enter my FTP credentials often has other usernames showing in the username field. I’m quite certain that this is showing the last person to log in, although there have been a few instances where it was not changed and I know that someone else has logged in so if that is the reason it’s intermittent.

Last week someone tried to register for my site and the address fields were prepopulated with another user’s details. This does not always happen, in fact only one person has alerted us to it and sent us a screenshot showing another user’s address details.

I’m at a loss as to what might be causing this and how to go about troubleshooting the issue. I’ve been unable to knowingly replicate it although I’ve seen enough to be confident that it is indeed happening.

Has anyone else encountered this problem? How did you solve it? If you haven’t encountered it, can you suggest the next steps for me?