Differences between CRAN osx and Travis xcode7.3 😫

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I’m running tests on macOS to explain why my tests fail on CRAN. Due to my latest check log my tests are failing on CRAN for a function writing html reports (https://www.r-project.org/nosvn/R.check/r-release-osx-x86_64/RTest-00check.html) (see log extract below)

But the tests go through in Travis-CI where I used xcode7.3+ xquartz, tcl-tk, freetype, tlmgr install framed titling
(https://travis-ci.org/zappingseb/RTest/jobs/475217824 - see log extract below. Additional R-packages are pkgdown and covr.

Does anybody have an explanation for this issue? I basically want to find out where my warning comes from.

CRAN

using R version 3.5.2 (2018-12-20)

using platform: x86_64-apple-darwin15.6.0 (64-bit)
...
    checking tests ... [8s/8s] ERROR
    Running ‘run_test.R’ [8s/8s]
    Running the tests in ‘tests/run_test.R’ failed.
    Last 13 lines of output:
...
  Error: Test failed: 'RTestCollection write HTML summary'
  * `writeExecSummary.html(...)` produced warnings.
  * `writeExecSummary.html(...)` produced warnings.

TRAVIS

    using R version 3.5.2 (2018-12-20)

    using platform: x86_64-apple-darwin15.6.0 (64-bit)
 ...
     Running ‘run_test.R’ [12s/14s]
    OK

.travis.yml for additional information

    os: osx
    osx_image: xcode7.3
    before_install:
      - brew update
       # solution from https://github.com/travis-ci/travis-ci/issues/8762
      - brew cask reinstall xquartz
      - brew install tcl-tk --with-tk
      - brew link --overwrite --force tcl-tk; brew unlink tcl-tk
      - brew install freetype 
      - brew link --overwrite freetype
      - sudo tlmgr install framed titling
    # solution from https://github.com/travis-ci/travis-ci/issues/7313
    before_script:
      - "export DISPLAY=:99.0"
      - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then ( sudo Xvfb :99 -ac -screen 0 1024x768x8; echo ok ) & fi
    script:
       - R CMD build . --compact-vignettes=gs+qpdf
       - R CMD check *tar.gz --as-cran