Roundcube: Server Error! Error no. [700]

Symptoms

When attempting to write an email in Roundcube, you receive the following error.

Server Error! Error no. [700]

Description

The error is caused by the address book in the "Store trusted senders in" setting not existing. Setting the value to an existing address book will correct the error.

Workaround

1. Log into Roundcube.
2. Navigate to "Settings / Preferences / Contacts."
3. Select another address book from the "Store trusted senders in" dropdown menu.
4. Click the "Save" button.