Mail messages cannot be filed in some iCloud mailboxes
I have about 50 iCloud mailboxes. Of those 50 there are suddenly half a dozen that will not accept messages I'm trying to file. Not just some messages, but all messages bounce back with the following words: "Unable to move messages: The message could not be moved to the mailbox ****" This seems to happen with any mailbox with APPLE or MAC in its name, but there are several others that behave in the same way. The mailboxes' content varies from dozens of messages to just a few. I can find no common thread at all. Fortunately I can file the problem emails on my iMac, but the situation renders iCloud pretty much useless. Is this a known bug or am I once again unique in experiencing a weird bug?
iPad Air, iOS 9.1