Array to string conversion?

Fatal error: PHP notice in file /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/system/template/Template.class.php (199): Array to string conversion
Information:
error message: PHP notice in file /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/system/template/Template.class.php (199): Array to string conversion
error code: 0
file: /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/system/WCF.class.php (281)
php version: 5.6.30-0+deb8u1
wcf version: 1.1.10 pl 2 (Tempest)
date: Fri, 10 Feb 2017 21:00:29 +0000
request: /index.php?page=PMView&pmID=281&folderID=0&pageNo=1&sortField=time&sortOrder=DESC&filterBySender=0
referer: https://have-fun-community.de/index.php?page=PMList
Stacktrace:
#0 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/system/template/Template.class.php(199): WCF::handleError(8, 'Array to string...', '/var/www/vhosts...', 199, Array)
#1 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/system/event/listener/MessageSidebarFactoryTrophiesListener.class.php(57): Template->append('additionalSideb...', Array)
#2 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/system/event/EventHandler.class.php(131): MessageSidebarFactoryTrophiesListener->execute(Object(MessageSidebarFactory), 'MessageSidebarF...', 'init')
#3 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/system/event/EventHandler.class.php(156): EventHandler::executeInheritedActions(Object(MessageSidebarFactory), 'init', 'MessageSidebarF...', 'init@MessageSid...')
#4 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/data/message/sidebar/MessageSidebarFactory.class.php(70): EventHandler::fireAction(Object(MessageSidebarFactory), 'init')
#5 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/page/PMViewPage.class.php(136): MessageSidebarFactory->init()
#6 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/page/AbstractPage.class.php(90): PMViewPage->readData()
#7 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/page/PMListPage.class.php(106): AbstractPage->show()
#8 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/page/PMViewPage.class.php(255): PMListPage->show()
#9 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/page/AbstractPage.class.php(46): PMViewPage->show()
#10 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/util/RequestHandler.class.php(64): AbstractPage->__construct()
#11 /var/www/vhosts/have-fun-community.de/httpdocs/wcf/lib/util/RequestHandler.class.php(92): RequestHandler->__construct('PMView', Array, 'page')
#12 /var/www/vhosts/have-fun-community.de/httpdocs/index.php(8): RequestHandler::handle(Array)
#13 {main}

if (!empty($variable)) {
				if (isset($this->v[$variable])) {
					if (is_array($this->v[$variable]) && is_array($value)) {
						$keys = array_keys($value);
						foreach ($keys as $key) {
							if (isset($this->v[$variable][$key])) {
								$this->v[$variable][$key] .= $value[$key];
							}
							else { 
								$this->v[$variable][$key] = $value[$key];
							}
						}
					}
					else {
						$this->v[$variable] .= $value;
					}
				}
				else {
					$this->v[$variable] = $value;
				}
			}
		}
	}