Cannot add new contacts to account

In addition to my usual work account (.gov.uk) my colleagues and I have shared accounts we use to manage our distribution list and other partnership related work. For reasons unknown, we can no longer add new outlook contacts to our 'partnership' account where our distribution list is saved. Whether we use the app in browser or in the desktop app, it is impossible to start a new contact using the 'New contact' button or the dropdown - it simply doesn't respond. I have tried removing and readding the account, clearing cache and using old and new outlook to no avail. Has anyone else had this issue and have a fix? Thanks