To answer both of your questions - actually if you use a web farm database updater, you can disregard those errors since the web farm tasks are synchronized through database and WF server URLs do not matter.
In any case, there was a bug regarding this error while having automatically generated servers which was fixed in the 8.2.28 hotfix. Details of the bug are following:
Web farms - Automatically generated WF servers generating 'Invalid URI' error
When web farms were set to generate servers automatically with the URL notifications synchronization mechanism enabled, the server root URL didn't contain the http(s) protocol which could result in the following error in the event log: "Message: Invalid URI: The format of the URI could not be determined."
If you want to find some more information about how web farms work in Kentico 8.x, we have got a really useful Knowledge Base article about that from the leading developer of the web farm module - Kentico 8 Technology – Web farm synchronization.