It's a silly suggestion, but I'll throw it out anyway - do you have cash available in the brokerage account? Only thing I can think of is that the primary account is the brokerage instead of checking and there's not enough cash to cover the withdrawal and fee.
I'd hope the CS rep checked that too but you never know sometimes. I haven't encountered a foreign ATM that lets me pick which account to withdrawal from, and I recall being able to get a debit card for the brokerage account when I set it up. I think I declined and just requested for the checking but I just realized it's been about 5 years since I created it so not sure if my card does only one or both. (
where does the time go!) Last withdrawal - which was also a few years ago now - automatically came from my checking. And I just remembered I have the app installed and confirmed mine is only showing linked to checking.
And again, hope the rep double checked this too, but the card is unlocked? A locked card also blocks ATM withdrawals.