Can't update plugins

Suddenly I can’t update plugins on my WP site. I’m getting “failed to create directory” and the admin backend locks up in maintenance mode. I haven’t changed anything on the server, but did fairly recently update WP to the current (4.4) version. Seems like a permissions problem but it’s been running for 2 years and never had this problem before. Has something about directory paths/permissions changed in the latest WP version (4.4)? Or is there something else I should check?