Avoid "complaint", "problem" and, most of all, "you".
People take it that it's they who have the problem.
Customers are twitchy, impatient & quick to ire.
The word 'entitled' springs to mind, frequently.
They start from the premise that it's your fault that whatever they're trying to do isn't working for them.
We all know - anyone who's ever worked helpdesk - that it's usually something dumb the user did... but you can't make it sound like that's the case.
What is your problem?
is often used as a precursor to a pub brawl.
What exactly is your complaint?
has a similar connotation.
They are both defensive/aggressive openers, avoid them. They imply you are standing your ground, that the fault is the user's.
They just have the wrong tone for a customer service department.
Remove the personalisation entirely. It's not the customer's problem, it's the software/service/device that has an issue.
Issue is the new global determiner for any customer service rep.
It depersonalises; prevents the user from feeling you think it's their fault - even if it usually is ;)
What is the issue?
Simple, to the point, puts the blame nowhere, on no-one.
If you need to clarify you have correctly recognised their intent, then
So, to clarify, the issue is that...
which still manages to avoid apportioning blame.