I've had a problem with creating new labels for managing my PMs for a while (ever since I started using them,) but never been able to reliably reproduce it. Very simply, it seemed that some days when I tried to create a new label and click the 'Add New Label' button it totally ignored me whatever I did. The post was sent back to the server okay, but when the page refreshed there had been no changes to the list of labels, then next day, or next time I logged in or something, it worked fine. It's not a big problem. I only need to do it once a month or so, and it usually worked after a day or two.
I've now worked out what factor triggering the problem is, and I can reproduce both success and failure cases reliably; and it's quite simply this:
I can't create new PM labels if I connect to E using https. (I use https whenever possible, but occasionally get sent back to http without noticing through links.)
I'm using Firefox 15.0.1 (with a whole shed-load of plugins), fully updated on Ubuntu Oneiric. I'm not entirely sure if this is a client or server issue, but it does reproduce when starting firefox in safe mode.
Like I said, it's not a big problem, and this post is mostly to point out the easy workaround to anyone else who is getting frustrated by it; but if there is an easy fix somewhere ...