Having a very long tree of subfolders can lead to problems in a Windows environment. You will not be able to expand the custom child folder if the path to it exceeds 260 characters, i.e.
"C:\Program Files (x86)\Kerio\MailServer\store/mail/domain.com/user/INBOX/All PERSONNEL/Employee - Lay off-Terminate-Resign/History - Lay off Term Resign/2017/2017 03-15 Sherlock, Holmes/2017 03-13 John, Watson/2015 01-05 New Hire John, Doe/Spring-All-company-meeting"
This article provides instructions on how to resolve the above issue.
Windows Maximum Path length limitation is causing this issue.
Shorten the long path of that specific folder by changing the names of the subfolders manually from the OS level.
Re-index the affected user mailbox from the Webadmin.
Windows Maximum Path Length Limitation
Kerio Connect installed in Windows