Changes have been made in Git (master):

commit 179b03c8de08b84fc462b2fc912de5770e930b97
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Wed Jul 25 05:55:30 2012 -0600

     [mms] Fix expanding all mailboxes in dynamic view (Bug #11289).

  imp/docs/CHANGES   |    1 +
  imp/js/dimpbase.js |    4 +++-
  imp/package.xml    |    2 ++
  3 files changed, 6 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/179b03c8de08b84fc462b2fc912de5770e930b97

Thanks Michael, working like a charm now!

Changes have been made in Git (develop):

commit 179b03c8de08b84fc462b2fc912de5770e930b97
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Wed Jul 25 05:55:30 2012 -0600

     [mms] Fix expanding all mailboxes in dynamic view (Bug #11289).

  imp/docs/CHANGES   |    1 +
  imp/js/dimpbase.js |    4 +++-
  imp/package.xml    |    2 ++
  3 files changed, 6 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/179b03c8de08b84fc462b2fc912de5770e930b97

Hi,

I enter imp and immediately click on "Folder actions" -> "Expand all".
-> Nothing happens.

I repeat it two more times and then it expands all folders. It seems 
to be timing related: If I wait long enough between the first and the 
second click, it will expand all folders on the second invocation.

Background info on the used IMAP account:
Around 68 folders, some of them are rather large (50.000+ messages).
Hierarchy is three or four folders deep.

I've also configured IMP to check for new mail every five minutes and 
to check all folders.

Maybe the "initial mail status check" is blocking the expand operation?

Thomas