Calendar items missing after 3.4 upgrade

[dsp3]

I have managed to complete the upgrade and make necessary changes to 
config files. However, 1 item I can't figure out. 2 users are missing 
all items from their calendars. They are present in imap folders and 
calendars are listed and subscribed, but no items are displayed. I have 
tried /usr/lib/cyrus-imapd/reconstruct -r -f mailboxname to rebuild the 
mailboxes of the users, but this didn't help.
What else to check?