There is a bug with the new html caching in oscailt 3.4 where sometimes the read back of the cached files fails and a blank page is displayed.
This is probably related to file locks and the response from the fopen probably fails and nothing is read back.
The solution was to use the function file_get_contents() instead of file() to read the full file. The former reads the file to a string while the later to an array. It is likely the former is quicker and does not suffer from the file lock problem since inherently it is a read operation
The fix is in version 1.9 of the index.php