Here's a slightly obscure problem I came across with a fresh build of K2 4.5.

The K2 workspace site threw HTTP 500 errors when I tried to view it - with an error like

Cannot add duplicate collection entry of type 'add' with unique key attribute 'value' set to 'negotiate'

When I looked in the web.config, for the Workspace site, I saw that the authentication config was slightly mis-configured:

<windowsAuthentication
enabled="true" useKernelMode="true"
useAppPoolCredentials="true">

                    <providers>

                        <remove
value="NTLM" />

                        <add
value="Negotiate" />

                        <add
value="NTLM" />

                    </providers>

 
</windowsAuthentication>

I added a <remove value="Negotiate" /> as the first sub-element of the <providers> tag - and everything was magically good.

I needed to do this in the following places:

C:Program Files (x86)K2
blackpearlWorkspaceSiteweb.config

C:Program Files (x86)K2
blackpearlWebservicesRuntimeServicesweb.config

C:Program Files (x86)K2
blackpearlWebservicesViewFlowweb.config

I'm still a bit puzzled why I got this issue (and no-one else has, apparently).  For various reasons I had to build and rebuild a number of K2 servers in 4.5 and 4.6, and came across this issue probably 60% of the time.  Once I knew what to do to fix it, it wasn't that painful, though.