In case it catches anyone else out...

Debian stretch/testing upgraded php7.0 to 7.0.3-6 on 25 Mar. In doing so it removed php-xml from the core package and so, for me, selfoss stopped pulling in new articles. (Sefloss stated 'finished' after a manual update - no obvious error messages - but didn't retrieve any new articles.) A manual install of php-xml (or php7.0-xml) was necessary to restore the selfoss functionality.