ProcessMap seems to work, but I get an error page when trying to open the Meridium Link Tool. Why?

In version 2.2.1 and earlier ProcessMap could not handle uiUrl on the format used in EPiServer CMS R2.
I.e. uiUrl="~/secret/UI/"

A workaround is to change this to uiUrl="http://host/secret/UI/". Remember to also change the Virtual Path Provider in the same way.
<add name="UI" virtualPath="/secret/UI/" .....