[horde] Install of imp with composer not possible

[Ferdinand Gruber]

I tried a new install of Horde with composer because I am not able to 
get my old Horde to work again since I upgraded to PHP8

The installation process using composer was finished without any problem.
Then I copied "/var/www/html/horde/web/horde/config/conf.php.dist" to 
"/var/www/html/horde/web/config/conf.php".

After that I tried to install imp.

composer require horde/imp

Now I get this errors from composer

Using version ^6.1 for horde/imp
./composer.json has been updated
Running composer update horde/imp
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

    - horde/imp[6.1.5, ..., 6.2.18] require pear-pear.horde.org/horde >=5.0.0 at stable,<=6.0.0alpha1 at stable ->
      could not be found in any version,  there may be a typo in the package name.
    - horde/imp[6.2.21, ..., 6.2.27] require pear-pear.horde.org/horde ^5 ->
      could not be found in any version, there may be a typo in the package name.
    - horde/imp[6.2.19, ..., 6.2.20] require pear-pear.horde.org/horde ^5 at stable ->
      could not be found in any version, there may be a typo in the package name.
    - horde/imp[6.2.0alpha1, ..., 6.2.0RC1] require pear-pear.horde.org/horde >=5.0.0,<=6.0.0alpha1 ->
      could not be found in any version, there may be a typo in the package name.
    - Root composer.json requires horde/imp ^6.1 -> satisfiable by horde/imp[6.1.5, ..., 6.2.27].

Systeminfo:
composer is installed on system level.
System: openSUSE Linux 15.4 with apche2 and PHP8

-- 
Regards from Austria
Ferdinand Gruber