Changes have been made in Git (FRAMEWORK_5_2):

commit 3b986c3d83fcea9c7c800232897c962810d38315
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed, 06 Jan 2016 13:37:18 -0500

Bug: 14216 Attempt to report back meaningful results.

For non-select statements, report back number of rows affected.

  M admin/sqlshell.php

https://github.com/horde/base/commit/3b986c3d83fcea9c7c800232897c962810d38315

Changes have been made in Git (FRAMEWORK_5_2):

commit 157c4adcbd56117142c2fba88bb10d775e62f9c1
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Jan 6 13:35:21 2016 -0500

     Bug: 14216 Attempt to report back meaningful results.

     For non-select statements, report back number of rows affected.

  horde/admin/sqlshell.php |   47 +++++++++++++++++++++++++++++++++++++++++----
  1 files changed, 42 insertions(+), 5 deletions(-)

http://github.com/horde/horde/commit/157c4adcbd56117142c2fba88bb10d775e62f9c1

Changes have been made in Git (master):

commit b754ca323b06f94892b20e285a04cfceaac57b2c
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Jan 6 13:35:21 2016 -0500

     Bug: 14216 Attempt to report back meaningful results.

     For non-select statements, report back number of rows affected.

  horde/admin/sqlshell.php |   47 +++++++++++++++++++++++++++++++++++++++++----
  1 files changed, 42 insertions(+), 5 deletions(-)

http://github.com/horde/horde/commit/b754ca323b06f94892b20e285a04cfceaac57b2c

There really isn't an easy solution here because of the way the 
Horde_Db package is structured. The best solution, IMO, would be to 
attempt to simply sniff out the type of query being performed and call 
the appropriate method. I should note that your UPDATE statement most 
likely did work. The issue is in the code that reports the result set 
back to the sqlshell controller.

The issue is we issue a $db->select() call from the sqlshell, which 
obviously assumes the SQL is a SELECT.  I see that the execute method 
is deprecated, so we need to check the return type before moving on.

Using the SQL Shell in the admin webinterface: 
Administration->Configuration->SQL Shell

Fails with an exception upon UPDATE-query execution.
Example query:
UPDATE imp_sentmail SET sentmail_who=1 WHERE sentmail_who=1


Log error mesage:
HORDE: [horde] Call to a member function fetch_array() on boolean [pid 
12311 on line 50 of "/usr/share/php/Horde/Db/Adapter/Mysqli/Result.php"]

Webinterface exception message:
ErrorException Object
(
     [message:protected] => Call to a member function fetch_array() on boolean
     [string:Exception:private] =>
     [code:protected] => 0
     [file:protected] => /usr/share/php/Horde/Db/Adapter/Mysqli/Result.php
     [line:protected] => 50
     [trace:Exception:private] => Array
         (
             [0] => Array
                 (
                     [function] => catchFatalError
                     [class] => Horde_ErrorHandler
                     [type] => ::
                     [args] => Array
                         (
                         )

                 )

         )

     [previous:Exception:private] =>
     [severity:protected] => 1
     [logged] => 1
)