Error in IMAP command UID SEARCH: Unknown argument

[snowfly]

One of my clients has reported they are unable to run searches in their IMAP mailbox using roundcube (1.1.4).

When they try and search in their Inbox (e.g. for 'office'), they get the error:
Server Error: UID SEARCH: Error in IMAP command UID SEARCH: Unknown argument OFFICE

If this user searches in any other folder in their mailbox, roundcube search works fine.

Server is running dovecot 2.1.7
I took a backup of the mailbox, deleted it, recreated it, then copied all the emails back into it, but still had an issue.

I have over 600 mailboxes hosted, and this is the only user that has reported this issue.

[SKaero]

Enable imap_debug and post the log when the problem occurs.