From the anecdotes I've heard, it sounds like the support staff is just wildly insufficient to handle all the issues that crop up with a major change like this. Initially the government switched over a trial group of (iirc) something like 40k employees. There were a bunch of problems with people not being paid or being paid the wrong amounts, but for some incomprehensible reason (perhaps they wanted to ditch the old system to focus on fixing the new one or something crazy like that?) they switched over everyone before fixing those.

Now, I personally know people who've been getting incorrect pay for over a year, who submit a complaint to their HR, who pass it up the chain, where... nothing seems to happen. Presumably because the 500 people or whatever it is tasked to deal with this are literally dealing with tens of thousands of issues, many of which aren't trivial changes (and so block the many that actually are trivial changes, but aren't even being seen).

Basically it was released before it was ready, initial signs of issues were ignored, and now the human side of the system is overwhelmed. It's kind of like when a web server gets overwhelmed with traffic, and it isn't set to reject at a reasonable threshold, so things start getting queued and timing out, people start refreshing pages, and it just snowballs until the whole thing goes down. Which apparently is what has now happened.