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After upgrading Pidgin and Sipe on Debain wheezy I came across:

Failed to authenticate to server

Desipe my password having not changed.

It turns out you need to disable "Use Single Sign-on" in the Advanced properties.

The Sipe plugin's website has a good FAQ dealing with problems therein: http://sourceforge.net/p/sipe/wiki/Frequently%20Asked%20Questions/