My question is in the OP.

[The Chosen One]

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

 

[Jordan]

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

 

[The Chosen One]

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]

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

 

[GTPorsche]

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

It means the thread has been locked.

 

[Aphelion]

It means the thread has been locked.

...but it's still open for other replies?