[horde] Fatal error when printing email

[Louis-Philippe Allard]

Hello,  trying to print an email by clicking on the Printer icon in the
email's header, I quickly see a window that pops up and immediately
disappear and the print job gives:

   A fatal error has occurred
Horde_Text_Filter: Class definition of csstidy not found.
1. Horde_Core_Factory_TextFilter->filter()
/var/www/htdocs/horde4/imp/view.php:287
2. Horde_Text_Filter::filter()
/usr/lib/php/Horde/Core/Factory/TextFilter.php:71
3. Horde_Text_Filter::factory() /usr/lib/php/Horde/Text/Filter.php:66
Details
The full error message is logged in Horde's log file, and is shown below
only to administrators. Non-administrative users will
not see error details.
Horde_Text_Filter_Exception Object
(
    [_previous:private] => 
    [details] => 
    [message:protected] => Horde_Text_Filter: Class definition of csstidy
not found.
    [string:private] => 
    [code:protected] => 0
    [file:protected] => /usr/lib/php/Horde/Text/Filter.php
    [line:protected] => 43
    [trace:private] => Array
        (
            [0] => Array
                (
                    [file] => /usr/lib/php/Horde/Text/Filter.php
                    [line] => 66
                    [function] => factory
                    [class] => Horde_Text_Filter
                    [type] => ::
                    [args] => Array
                        (
                            [0] => csstidy
                            [1] => Array
                                (
                                    [ob] => 1
                                    [preserve_css] => 
                                )
                        )
                )
            [1] => Array
                (
                    [file] =>
/usr/lib/php/Horde/Core/Factory/TextFilter.php
                    [line] => 71
                    [function] => filter
                    [class] => Horde_Text_Filter
                    [type] => ::
                    [args] => Array
                        (
                            [0] => * {  padding: 0; margin: 0;}body, input,
select, textarea, .inProgress { font-family: Geneva,Arial,"DejuVu
Sans",sans-serif;}bo
                            [1] => Array
                                (
                                    [0] => csstidy
                                )
                            [2] => Array
                                (
                                    [0] => Array
                                        (
                                            [ob] => 1
                                            [preserve_css] => 
                                        )
                                )
                        )
                )
            [2] => Array
                (
                    [file] => /var/www/htdocs/horde4/imp/view.php
                    [line] => 287
                    [function] => filter
                    [class] => Horde_Core_Factory_TextFilter
                    [type] => ->
                    [args] => Array
                        (
                            [0] => * {  padding: 0; margin: 0;}body, input,
select, textarea, .inProgress { font-family: Geneva,Arial,"DejuVu
Sans",sans-serif;}bo
                            [1] => csstidy
                            [2] => Array
                                (
                                    [ob] => 1
                                    [preserve_css] => 
                                )
                        )
                )
        )
)

    Any clue?
    Thanks!