Fatal error: PHP error in file /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/UserGuestbookListPage.class.php (29): Non-static method UserGuestbookData::countEntries() should not be called statically, assuming $this from incompatible context

Information:

error message: PHP error in file /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/UserGuestbookListPage.class.php (29): Non-static method UserGuestbookData::countEntries() should not be called statically, assuming $this from incompatible context
error code: 0
file: /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/system/WCF.class.php (255)
php version: 5.6.31
wcf version: 1.0.11 RC 1 (Horizon)
date: Sat, 16 Dec 2017 12:40:50 +0000
request: /index.php?page=UserGuestbookList&s=275baa40452fe0f908f1217e7475f7b719f07898
referer:

Stacktrace:

#0 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/UserGuestbookListPage.class.php(29): WCF::handleError(8192, 'Non-static meth...', '/customers/d/b/...', 29, Array)
#1 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/MultipleLinkPage.class.php(79): UserGuestbookListPage->countItems()
#2 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/MultipleLinkPage.class.php(67): MultipleLinkPage->calculateNumberOfPages()
#3 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/UserGuestbookListPage.class.php(46): MultipleLinkPage->readData()
#4 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/AbstractPage.class.php(74): UserGuestbookListPage->readData()
#5 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/UserGuestbookListPage.class.php(72): AbstractPage->show()
#6 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/AbstractPage.class.php(36): UserGuestbookListPage->show()
#7 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/page/UserGuestbookListPage.class.php(21): AbstractPage->__construct()
#8 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/util/RequestHandler.class.php(63): UserGuestbookListPage->__construct()
#9 /customers/d/b/c/liberationofchaos.de/httpd.www/wbb309rc1/lib/util/RequestHandler.class.php(91): RequestHandler->__construct('UserGuestbookLi...', Array, 'page')
#10 /customers/d/b/c/liberationofchaos.de/httpd.www/index.php(8): RequestHandler::handle(Array)
#11 {main}