Fatal error adding new block

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This site has been running fine for some time. There have been no edits to it for at least a month. No updates.

With no (apparent) changes to the site, it returns the following error & warning when attempting to add a block. There is no problem editing an existing block on a page.

Did a manual reinstall but that didn't correct the problem.

Any ideas?

Warning: require_once(/var/www/bystanders.org.uk/web/packages/groupdocs_viewer/blocks/groupdocs_viewer/controller.php): failed to open stream: No such file or directory in /var/www/clients/client1/web2/web/concrete/core/libraries/loader.php on line 217
...
Fatal error: require_once(): Failed opening required '/var/www/bystanders.org.uk/web/packages/groupdocs_viewer/blocks/groupdocs_viewer/controller.php' (include_path='/var/www/bystanders.org.uk/web/libraries/3rdparty:/var/www/clients/client1/web2/web/concrete/libraries/3rdparty:.:/usr/share/php:/usr/share/pear') in /var/www/clients/client1/web2/web/concrete/core/libraries/loader.php on line 217