Maybe a fraudulent return was filed or maybe they went online and filed a "simple return" to get the EIP.  This is going to be a rough year with all the EIP nonsense.  And we won't be done with it until the 2020s are filed.

If you're just filing an extension and can't get it to go through, you'll probably end up having to paper file the return.  I would try having the taxpayer go to IRS DirectPay and make an extension payment there.  If you're not planning on making a payment with extension, I'd have them pay $1 just to get a DirectPay extension in the system.  And don't wait until the last minute.  It has a nasty habit of crashing on 4/15 and 10/15.

Rick