I'm really not sure how to go about fixing an issue without knowing the error. The person whose facing the issue isn't cooperative at all.

If a user is being an arse, first decide if you actually have to fix it. Do they have a support contract with you? Are they particularly influential and therefore have a high chance of damaging your reputation? If not, just start ignoring them. Not worth the time.

If you actually need to get to the bottom of the issue though, at this point I usually insist on a call with screenshare where they take you through the problem. At least then you can see for yourself what's happening (even if you have to put up with the user moaning away in the process.)