I hadn't thought of contacting my security software provider direct before, thinking any emails I sent them would just disappear inside a corporate holding pattern and nothing would be done.
However, as I was going through the issue described above, I figured we had nothing to lose and clicked on the 'contact us online' link (something like that) for Trend Micro, which put me on a kind of direct chat thing with one of their operators in which I outlined the problem and they directed me to try a couple of simple things to narrow it down.
They followed up with a series of emails and, as I said, kept at it until it was fixed. Quite a positive experience - don't know if whichever company you use would do the same, PPrune Pop, but it might be worth a go if all else fails.