If core files are generated during the run agent cannot upload them. There is no specific reason in the log, but most likely cause is lack of permissions.

https://testresults.qt.io/logs/qt/qtbase/37e26838f723a07dd6012fa3c5e9275b68bb118e/MacOSMacOS_10_14x86_64MacOSMacOS_10_14x86_64Clangqtci-macos-10.14-x86_64-8113f7UseLegacyInstructions/f0c5638c7fd74654a56b0ff7442a06eeab71a677/build_1602498144/log.txt.gz