I had the same problem. After applying the two patches in this thread 
things started to work
apart from some moaning about the Calender Database.

The problem turned out to be that I had used 
.../scripts/sql/create.mysql.sql rather than
the correct .../scripts/sql/groupware.sql, and so was missing quite a 
few of the database tables.

The MDB2_Error::execute() stuff was just a red herring.

Not until we know what the actual error is...

There have been a few more error checks and error logging added to the 
code that was producing the oringinal fatal.  If the someone can try 
this again after updating the code and report back any errors either 
displayed on in the horde log, it would be helpful.
What makes you thing the MDB2 package is not found?  The problem 
reported in this ticket is that an MDB2_Error is being accessed as if 
it were an MDB2_Result or similar object...


If I try to use mysql (not mysqlI)

These are exactly what they say. You are attempting to create a 
database that already exists.

hello,

I got exactly the same error with lastest rc4 version. Is there a way 
to fix it?

I've tried several ways, but still have no clue why the script can not 
find the MDB2 package.  If I try to use mysql (not mysqlI) setup.php 
returns also another error:

Loading database module...
[  WARN  ] Database horde exists already, skipping.
Creating database...
[ ERROR! ] Creating the tables failed. Please see the manual at docs/INSTALL
            for how to setup the database manually. Error messages:
MDB2_Schema Error: unknown error
the database was only partially created (MDB2 Error: unknown error 
(_doQuery: [Error message: Could not execute statement]
[Last executed query: ALTER TABLE horde_datatree ADD PRIMARY KEY 
(datatree_id)]
[Native code: 1068]
[Native message: Multiple primary key defined]
))

Thanks,
Markus

Hello! I've got exactly the same problem here.
I'm using PHP 5.2.0. mysql driver and mdb2 installed using pear.

Added more error checking:
http://cvs.horde.org/diff.php?f=framework%2FShare%2FShare%2Fsql.php&r1=1.1.2.27&r2=1.1.2.28&t=unified

After installing that patch I now see:

Call to undefined method MDB2_Error::execute() in 
/www/webdocs/webmail/lib/Horde/Share/sql.php on line 869

http://lists.horde.org/archives/cvs/Week-of-Mon-20080526/079142.html

We now at least catch and return this error.  Hopefully you'll now see 
some more useful information in the logs or notifications.

I've done a parallel install of 1.1, leaving my working 1.0.6 install alone.

I followed the installation notes, installing everything that's marked
as required and almost everything that was recommended.  I've
configured it to use MySQL, like my 1.0.6 install (I'm using a
different database, horde11 instead of horde, so I can run in
parallel).  However, I'm getting the following error after trying to
login:

Fatal error: Call to undefined method MDB2_Error::execute() in
/www/webdocs/webmail/lib/Horde/Share/sql.php on line 554

I've installed MDB2 and the mysql driver (version 1.4.1), PHP 5.2.5,
Apache 2.2.8 and Horde Webmail 1.1.  It all works fine with 1.0.6.
I'm obviously missing something obvious, as I've not seen anybody else
report this.