Problem: during the export, XWiki needs the Velocity Engine.
In DefaultVelocityManager#getVelocityEngineCacheKey() tries to get the "macros.vm" of the current skin:
But getSkinFile actually returns the URL to a temp file, because during the export action, the FileSystemURLFactory is used:
Here, for the "macros.vm" special case, we should use the standard URL factory.
Because of that problem, XWiki fallbacks to the "macros.vm" of the default base skin, even if the current skin has it own macros.vm.