I've installed the plugin and I get no errors - however I have to give the user the capability of Edit Others pages to allow them to edit the page/post - this seems to overwrite the UAM setup.
e.g. I have a user called test
I put them in the Midlands UAM group
The Midlands group is set to read All, Write only the group
I create a Page and put it into the Midlands Group
If the user does not have Edit Other Pages ticked in the User Role Editor - then they can't edit the page
If the user does have Edit other Pages ticked then they can edit the page - but they can also edit any other page - whether it's in the Midlands group or not
I created another group called South - and assigned a Page to that group - but the Midlands user can edit that page as well.
I also have Advanced Access Manager installed and User Role Editor plugins - Though I have tried with these both disabled too