"We’ve interrupted your sharing activity" error when sharing content, and older links don't work.

For the last couple of weeks we've been unable to share files, and of course prior shares are no longer working. We upload files each week, no particular change about those (zip file and PDFs), and we're reasonably sure there's nothing dodgy about/in those files.

There seems to be no way of finding out what's triggered this sharing ban. The bot only suggests running through a security review, which was done and didn't help. 

We're on the Basic plan (non-profit community newspaper so not looking for more expenses) which means no direct Support is available.

Any ideas on how we can find the trigger and/or have the ban lifted? I'm sure this is a false-positive......