Well, after a quick search there apparently is a Chromium (and therefor Opium) bug: https://code.google.com/p/chromium/issues/detail?id=178380
May be connected with Adblock Plus (do you use that?) and may be worked around with a "cache killer" extension (https://chrome.google.com/webstore/detail/cache-killer/jpfbieopdmepaolggioebjmedmclkbap).
