When searching in Webmail, nothing is shown. A small red error appears after some time.
Dovecot reads from its index files when it searches emails in webmail. The issue that webmail exhibits, in this case, may be caused by a corrupted Dovecot Index Cache. To address this if you're using maildir for storage, you can remove the Dovecot Index files with:
/scripts/remove_dovecot_index_files --user cpuser
Replacing "cpuser" with your username. In doing so, you will remove the current Dovecot Index files. Refreshing webmail will regenerate them.