Found the problem!
While uploads for attachments are fine, the contacts list (and likely any other import/export plugin like stuff) defaults to the PHP upload_tmp_dir value, which is /srv/example.com/php_tmp, which is fine for the regular sites hosted, but as roundcube is aliased, it breaks as it can’t access the file which was uploaded.
I should have fix for sympl-webmail pushed out soon to the testing branch, and I’ll fast-track it to stable.