[horde] Mail folders migration & Horde theme per user

[balia]

Thank you Luis Felipe; your solution worked.
For others who may encounter this problem on Horde 5.2.20, I didn't  
have "Show all folders" but "Show all Mailboxes".

However regarding "Advanced Preferences", I don't have this option at all.
There is no "Show advanced preferences" under Preferences > Mail or  
any other tab.
Do I need to enable it in conf.php?

Also, the user prefers the old Horde theme and doesn't like the Horde 5 theme.
Can I change the theme per user? How can I do that?

Best.

--