Would be only a hint I think ...
Found out, if I deactivate the access check in PreviewModel, than a Author is able to watch an older article via versions.
Don't understand, why the access-check is not working for a preview - the access-check for CompareModel is looking quite similar - but there are no access-errors for an author.
Changed (deactivated) lines in
administrator/components/com_contenthistory/src/Model/PreviewModel.php
`

   $user = Factory::getUser();

    // Access check
    //if (!$user->authorise('core.edit', $table->item_id) && !$this->canEdit($table)) {
    //    throw new NotAllowed(Text::_('JERROR_ALERTNOAUTHOR'), 403);
    //}` 

Hopefully somewhere is reading this with more understanding how that all is working...