When conducting a search within webmail, an error is returned stating:
error in IMAP command uid searc: unknown argument
This behavior can result in corrupted Roundcube user preferences, specifically in the search_mods array, which stores the selected search options for each folder (To, From, Subject, etc.). Some folder names or other bad search options may be stored as search preferences for a specific folder, resulting in the error occurring only when searching that folder.
The suggestion is to clear the preferences which should return the normal functionality. This may be done with the following command:
sqlite3 /home/$user/etc/$domain/$mail_user.rcube.db 'UPDATE users SET preferences = "" WHERE username = "firstname.lastname@example.org"'
*Please be sure to update the example path and user