Failed to determine definition for Feature with ID '<guid>'. Skipping this feature for element querying consideration.This message is caused by a feature that has been removed from the environment, without being properly deactivated before removal. SharePoint still has a reference to the feature, so it tires to activate the feature. But because it doesn't exist anymore, it will skip the feature.
Last week I ran into a tool which can scan your environment for such a "faulty feature". This tool is called the "SharePoint Feature Administration and Clean Up Tool" and can be found on Codeplex.
When you have started the tool, it contains a button called "Find Faulty Feature in Farm", which starts the scan for faulty features.
Have fun with this great tool!