After migrating an extension of the dd_googlesitemap from T3 4.5 to 6.2, I noticed a very strange behavior which occurred after every cache clear in the install tool. Every call of index.php?eID=dd_googlesitemap or a custom sitemap index.php?eID=dd_googlesitemap&sitemap=customsitemap resulted in the following error:
#1: PHP Warning: Invalid argument supplied for foreach() in [...]/typo3_src-6.2.3/typo3/sysext/core/Classes/Utility/RootlineUtility.php line 269
The error is misleading, because it seems to have nothing to do with the skipRootlineCheck option the extension provides. The error simply disappears when accessing the frontpage after the cache clear, just before accessing the sitemap.