The issue happens especially to new accounts. That means, the one working "Author" account is the oldest one. But if you compare the two, there seems to be no difference (Editor settings, etc.)
In the meantime, i narrowed down the problem and realized that this probably isn't a TZ portfolio+ issue. The authors are also not able to create new normal articles outside TZ. So never mind, i'll look further for help. In case you have any idea, i'll attach below the error message the users get when turning on the Joomla-Debug mode. It rather seems to be a problem with community polls and/or jcomments, but i am not yet able to read these messages properly :-).
Call to undefined method JEventDispatcher::getContent()
Call stack
# Function Location
1 () JROOT/plugins/editors-xtd/jcommentson/jcommentson.php:29
2 plgButtonJCommentsOn->onDisplay() JROOT/libraries/joomla/event/event.php:70
3 JEvent->update() JROOT/libraries/joomla/event/dispatcher.php:160
4 JEventDispatcher->trigger() JROOT/libraries/src/Application/BaseApplication.php:108
5 Joomla\CMS\Application\BaseApplication->triggerEvent() JROOT/components/com_communitypolls/layouts/default/poll/answers.php:242
6 include() JROOT/plugins/system/t3/includes/joomla4/FileLayout.php:125
7 Joomla\CMS\Layout\FileLayout->render() JROOT/administrator/components/com_communitypolls/helpers/communitypolls.php:355
8 CommunityPollsHelper::renderLayout() JROOT/plugins/content/polls/polls.php:145
9 replace_poll_tags()
10 preg_replace_callback() JROOT/plugins/content/polls/polls.php:49
11 plgContentPolls->onContentPrepare() JROOT/libraries/joomla/event/event.php:70
12 JEvent->update() JROOT/libraries/joomla/event/dispatcher.php:160
13 JEventDispatcher->trigger() JROOT/libraries/src/Application/BaseApplication.php:108
14 Joomla\CMS\Application\BaseApplication->triggerEvent() JROOT/modules/mod_tz_portfolio_plus_articles/helper.php:199
15 modTZ_Portfolio_PlusArticlesHelper::getList() JROOT/modules/mod_tz_portfolio_plus_articles/mod_tz_portfolio_plus_articles.php:31
16 include() JROOT/plugins/system/t3/includes/joomla4/ModuleHelper.php:201
17 Joomla\CMS\Helper\ModuleHelper::renderModule() JROOT/libraries/src/Document/Renderer/Html/ModuleRenderer.php:98
18 Joomla\CMS\Document\Renderer\Html\ModuleRenderer->render() JROOT/libraries/src/Document/Renderer/Html/ModulesRenderer.php:47
19 Joomla\CMS\Document\Renderer\Html\ModulesRenderer->render() JROOT/libraries/src/Document/HtmlDocument.php:501
20 Joomla\CMS\Document\HtmlDocument->getBuffer() JROOT/libraries/src/Document/HtmlDocument.php:793
21 Joomla\CMS\Document\HtmlDocument->_renderTemplate() JROOT/libraries/src/Document/HtmlDocument.php:567
22 Joomla\CMS\Document\HtmlDocument->render() JROOT/libraries/src/Application/CMSApplication.php:1044
23 Joomla\CMS\Application\CMSApplication->render() JROOT/libraries/src/Application/SiteApplication.php:778
24 Joomla\CMS\Application\SiteApplication->render() JROOT/libraries/src/Application/CMSApplication.php:202
25 Joomla\CMS\Application\CMSApplication->execute() JROOT/index.php:49