HTTP403 for posting *some* orders

For one asset, I’m starting to get HTTP403 while posting (in extended hours in case it matters) today. For another asset, I’m not getting that response.

What is the cause for HTTP403 usually in such circumstances with Alpaca?

Note that this problem didn’t arise for either asset during market hours. It literally started after 4PM ET. (And yes, I’m setting extended_hours=true).

Update:

The cause seems to be that the shares are locked because this order has been stuck at pending_cancel for 2+ days:

d9852d62-5790-4bdf-b1fc-75c226478425 TSLA SELL 10

I really wish I knew how (as a user/customer) we can fix orders stuck in pending_cancel state for days.

Anybody know someone to @ mention to take a look on the server side?