documentation build fails in Coin for Linux

It seems that for some reason the Linux documentation build fails in the Coin for RC6

Find "fatal error:" in the corresponding log.

Link:

http://10.212.0.100/coin/logview/tqtc-boot2qt/qtsaferenderer/5467e4566c8440b7c58aa1256b963f9656f94f34/LinuxUbuntu_18_04x86_64LinuxUbuntu_18_04x86_64GCCqtci-linux-Ubuntu-18.04-x86_64-c13652Documentation_OutOfSourceBuild/661956d08e53ba101307e8056b752f6d8c258de2/build_1611148002

excerpt from log:

 /tmp/qdoc_pch.OCbiag/QtSafeRendererDoc:1:10: fatal error: 'QtSafeRenderer' file not found /usr/lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/type_traits:111:14: error: base specifier must name a class

It is bit confusing that this refers to GCC 7.,5 although the machine should be using GCC 9.2.0.

But at the same time it works in the nightly test server.

Link:

http://10.204.0.5/1.2/2021-01-21.00.01.01/SafetyManual/qtsaferenderer-index.html