While working with an upgraded Sitecore build from 6.5 to 8.1 I have come across an issue with Path Analyzer that I thought worth sharing as I had trouble finding solutions to it on the internet.
After logging into Sitecore and clicking on the Path Analyzer button on the Launch Pad I saw this error:
To troubleshoot I tried accessing the pathanalyzer page to get the details of the error. The page lives on the path http://hostname/sitecore/admin/pathanalyzer.aspx
When I opened that page I saw a NullReferenceException error:
To solve this you need to enable the path analyzer client to communicate with the remote reporting server by making the following changes, as described here:
Enabled the file Sitecore.PathAnalyzer.RemoteClient.config and disable the files Sitecore.PathAnalyzer.Processing.config and Sitecore.PathAnalyzer.Services.RemoteServer.config.
After making this change the Path Analyzer started working again. Credits to Jose Neto for the help
The portuguese version of this post can be read here