Internal error in ApiFormatFeedWrapper::execute: Invalid feed class/item

Backtrace:

#0 C:\Inetpub\vhosts\kidney.de\coviki.org\cov\includes\api\ApiBase.php(1241): wfDebugDieBacktrace('Internal error ...')
#1 C:\Inetpub\vhosts\kidney.de\coviki.org\cov\includes\api\ApiFormatBase.php(374): ApiBase::dieDebug('ApiFormatFeedWr...', 'Invalid feed cl...')
#2 C:\Inetpub\vhosts\kidney.de\coviki.org\cov\includes\api\ApiMain.php(728): ApiFormatFeedWrapper->execute()
#3 C:\Inetpub\vhosts\kidney.de\coviki.org\cov\includes\api\ApiMain.php(387): ApiMain->printResult(true)
#4 C:\Inetpub\vhosts\kidney.de\coviki.org\cov\includes\api\ApiMain.php(342): ApiMain->executeActionWithErrorHandling()
#5 C:\Inetpub\vhosts\kidney.de\coviki.org\cov\api.php(115): ApiMain->execute()
#6 {main}