ERROR building: error unmarshalling testlib xml output

I'm trying to run QML tests of QtWebKit in Coin with Qt 5.15.1, but repeatedly get this error:

ERROR building: error unmarshalling testlib xml output ('C:\Users\qt\work\testresults\tests\webkitqml\qmltests622500556'): 'XML syntax error on line 39: expected attribute name in element'.

See https://testresults.qt.io/coin/api/results/qt/qtwebkit/252ad67942e7fe877910f01ebf1ad7692d6c85b3/WindowsWindows_10x86_64WindowsWindows_10x86_64Mingwqtci-windows-10-x86_64-17-180c45/25ce44dfd2853540d435c6ee185826d3a568969e/test_1600877217/log.txt.gz, https://testresults.qt.io/coin/api/results/qt/qtwebkit/252ad67942e7fe877910f01ebf1ad7692d6c85b3/WindowsWindows_10x86_64WindowsWindows_10x86_64Mingwqtci-windows-10-x86_64-17-180c45/25ce44dfd2853540d435c6ee185826d3a568969e/test_1600869985/log.txt.gz

There is no crash in the log. I cannot reproduce that XML error locally. It would be great if it was possible at least to extract that failing XML file to have a chance to work around a problem.