User password change in Roundcube fails

Any theories? Here’s the redacted log:

[26-Jun-2020 17:38:34 +0200]: <bbm3kset> DB Error: [8] attempt to write a readonly database (SQL Query: UPDATE users SET password='{SHA512-CRYPT}$6$/M36LoRHRUk6a.Te$2Cu5AE7EzjhBcw5leyCadtyI.xl7r4VoFQmYVAmkdXnIAaxHlyG43HlWABbQx6rb1CPLeIpxVQpBMBoaV8mO0' WHERE email='') in /usr/local/lib/roundcubemail/program/lib/Roundcube/rcube_db.php on line 543 (POST /mail/?_task=settings&_action=plugin.password-save)


Never mind; worked it out by comparing with my other installation. users.sqlite was not group www-data. Changing the group fixed it. How weird!