I got the same error, I just grabbed the mail and openedmail files from the 3.4.8 archive and dropped them into my 3.4.1 installation and the mail windows work fine now. I'm just patching 3.4.1 as needed to keep it unborked until the latest releases settle down and, more importantly, until I sort out what all I've added/changed since I put 3.4.1 in.
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