My question is in the OP.

The Chosen One · Apr 24, 2015

What does it mean when I have "Insufficient privileges" to reply to a thread, but it's still open for other replies?

Jordan · Apr 24, 2015

It means you have been banned from replying to that thread by a moderator.

The Chosen One · Apr 25, 2015

Jordan
It means you have been banned from replying to that thread by a moderator.

Oh, sometimes that happens on threads I haven't even replied to.

Jordan · Apr 25, 2015

Hmmm... Can you link to example threads where you see this?

GTPorsche · Apr 25, 2015

The Chosen One
Oh, sometimes that happens on threads I haven't even replied to.

It means the thread has been locked.

Aphelion · Apr 25, 2015

GTPorsche
It means the thread has been locked.

The Chosen One
...but it's still open for other replies?