Error on Windows server could not open relation base/xxx/xxx Permission denied

One of my clients is getting this problem occasionally. Actually, we can cause it to happen quite reliably by pasting certain text into a couple of fields, but the vast majority of text entered into the vast majority of fields causes no problem.

I've read enough to suggest that AV software might be the culprit. It has been said that it is not sufficient to exclude the database directory nor even to disable to AV protection, it has to be removed.

The problem is, their database server is also a file server. As a file server it must have AV protection. The server is running Windows Server 2003 I believe. It has RAID etc. My client's antivirus software is AVG (paid, not free).

Question: Is AV software still regarded as the likely culprit?

Question: If so, is any particular brand less likely to cause problems, more likely?

Question: Any other suggestions?

I'd had to tell my client to purchase more hardware because the database software I've recommended has a problem. I have a number of other clients using Postgres and nobody else has had any problem. Switching AV software wouldn't be such an issue.

Thanks.

John