MUC breaks on purge all caches - race condition when creating directories

I noticed a weird error in theme serving scripts:

Invalid permissions detected in $CFG->dataroot directory, administrator has to fix permissions.

Debug info: [cachedir]/cachestore_file/default_application/core_config can not be created, check permissions.
Error code: invaliddatarootpermissions

Stack trace:

• line 1264 of /lib/setuplib.php: invalid_dataroot_permissions thrown
• line 269 of /cache/stores/file/lib.php: call to make_writable_directory()
• line 278 of /cache/classes/factory.php: call to cachestore_file->initialise()
  *line 152 of /cache/classes/helper.php: call to cache_factory->create_store_from_config()
  *line 137 of /cache/classes/helper.php: call to cache_helper::initialise_cachestore_instances()
• line 234 of /cache/classes/factory.php: call to cache_helper::get_cache_stores()
  *line 183 of /cache/classes/factory.php: call to cache_factory->create_cache()
  *line 171 of /cache/classes/loaders.php: call to cache_factory->create_cache_from_definition()
• line 1408 of /lib/moodlelib.php: call to cache::make()
• line 741 of /lib/setuplib.php: call to get_config()
• line 595 of /lib/setup.php: call to initialise_cfg()
• line 122 of /theme/styles.php: call to require()