Unable to move email to a particular subfolder

I'm having trouble moving email to one particular subfolder of an email account. I've tried using both SquirrelMail and Thunderbird (which work fine for all other folders) with the same results.

I have plenty of storage space on the email account (using < 10MB / 200) and plenty of space on my entire account as well.

I can move emails into several other folders, but when I try to move into this one I get the following error:

ERROR: Could not complete request.
Query: COPY 609 "INBOX.Open Cases.Confidential"
Reason Given: [ALERT] COPY failed - no write permission or out of disk space.

Is there a per folder quota for email that I can change somewhere or is this a different problem?

Thanks in advance for any replies!

I don't use periods in my folders. Could that be it? Or is Confidential a subfolder of Open Cases?

-Scott

Confidential is a subfolder of Open Cases which is a subfolder of Inbox. The periods are how the error message formatted the relationship.