Changes have been made in Git (master):

commit f513e99ae26d27f13388b811f116ce07528155c5
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Mon Mar 3 12:12:33 2014 -0700

     Bug #12999: fix PHP warning

  imp/lib/Mailbox/SessionCache.php |    3 ++-
  1 files changed, 2 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/f513e99ae26d27f13388b811f116ce07528155c5

I've modified the code to show some more debug information:

---git diff---------------------------------------------
diff --git a/imp/lib/Mailbox/SessionCache.php 
b/imp/lib/Mailbox/SessionCache.php
index eb5466b..fa855af 100644
--- a/imp/lib/Mailbox/SessionCache.php
+++ b/imp/lib/Mailbox/SessionCache.php
@@ -81,6 +81,18 @@ class IMP_Mailbox_SessionCache implements Serializable
       */
      public function getAcl($mbox)
      {
+        if (is_null($this->_cache)) {
+            $e = new Exception();
+            Horde::log("TOMJ: null cache");
+            Horde::log($e->getTraceAsString(), 'INFO');
+        }
+        else if (is_null($this->_cache[$mbox])) {
+            $e = new Exception();
+            Horde::log("TOMJ: Dumping invalid cache entry");
+            Horde::log(print_r($this->_cache, true));
+            Horde::log($e->getTraceAsString(), 'INFO');
+        }
+
          if (!array_key_exists(self::CACHE_ACL, $this->_cache[$mbox])) {
              return false;
          }
------------------------------------------------

Here's the output:

------------------------------------
2014-02-28T09:36:30+01:00 INFO: HORDE [imp] TOMJ: Dumping invalid 
cache entry [pid 3913 on line 91 of 
"/datastore/DEVEL/horde/imp/lib/Mailbox/SessionCache.php"]
2014-02-28T09:36:30+01:00 INFO: HORDE [imp] Array
(
)
  [pid 3913 on line 92 of 
"/datastore/DEVEL/horde/imp/lib/Mailbox/SessionCache.php"]
2014-02-28T09:36:30+01:00 INFO: HORDE [imp] #0 
/datastore/DEVEL/horde/imp/lib/Mailbox.php(326): 
IMP_Mailbox_SessionCache->getAcl('INBOX')
#1 /datastore/DEVEL/horde/imp/lib/Mailbox.php(566): IMP_Mailbox->__get('acl')
#2 /datastore/DEVEL/horde/imp/lib/Basic/Mailbox.php(74): 
IMP_Mailbox->__get('readonly')
#3 /datastore/DEVEL/horde/imp/lib/Basic/Base.php(69): 
IMP_Basic_Mailbox->_init()
#4 /datastore/DEVEL/horde/imp/basic.php(34): 
IMP_Basic_Base->__construct(Object(Horde_Variables))
#5 {main} [pid 3913 on line 93 of 
"/datastore/DEVEL/horde/imp/lib/Mailbox/SessionCache.php"]
2014-02-28T09:36:30+01:00 WARN: HORDE [imp] PHP ERROR: 
array_key_exists() expects parameter 2 to be array, null given [pid 
3913 on line 96 of 
"/datastore/DEVEL/horde/imp/lib/Mailbox/SessionCache.php"]
------------------------------------

-> $this->_cache is still empty at the point of the call.

Still happens today :o)

2014-02-28T09:17:48+01:00 WARN: HORDE [imp] PHP ERROR: 
array_key_exists() expects parameter 2 to be array, null given [pid 
2744 on line 84 of 
"/datastore/DEVEL/horde/imp/lib/Mailbox/SessionCache.php"]

# git log --decorate=short
commit bf762c85b24d4614f6e4bad8df1a1c4a59a6258e (HEAD, origin/master, 
origin/HEAD, master)
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Thu Feb 27 13:05:47 2014 -0700

     Fix form type


--------------------------------------------

If it helps: I'm using the preconfigured 'advanced' server in backends.php
with 'hordeauth => true'. Only happens in the "basic" view.

For the sake of completeness, I've clear all horde caches
and made sure my git tree is clean.

This was fixed yesterday.

git HEAD issues the following warning in git HEAD of master:

2014-02-27T16:49:26+01:00 WARN: HORDE [imp] PHP ERROR: 
array_key_exists() expects parameter 2 to be array, null given [pid 
18634 on line 84 of 
"/datastore/DEVEL/horde/imp/lib/Mailbox/SessionCache.php"]