This is a strange issue that seems to come and go. Currently I am being able to reproduce it all the time, both using DEV and PROD environments.

steps to reproduce:

• Install ezpublish 5.4, upgrade it to 5.4.5 and SetEnv to "dev"
• Open the back end interface and select the Content Structure view
• Using the console, clear cache with php ezpublish/console cache:clear --env=dev --no-warmup
• Once the process completes, go to the browser and refresh the window (showing the Content Structure)
• Once refresh is complete, refresh the page once again (using CTRL+R)
  => The content tree will display an Internal error
  => Nothing will be rendered in dev.log or apache's error_log referring this error
  => A status 500 will be visible in apache's access_log for a get of an url like /content/treemenu/2/1458769396/1458826301/43b864bde60564a9abad0598b1d5d203
• if you refresh the page once more, the Internal error will be gone and won't be back until cache is cleared once more

Since nothing is written in the error_log I am suspect that whatever is failing is in a php function being called with @, but I could not find out which, so far...