The operation failed because the following URL does not correspond to a SharePoint Site collection : http:/sharepointsite/_layouts/IniWrkflIP.aspx

I got this error recently
"Error loading and running event receiver Microsoft.SharePoint.Workflow.SPWorkflowLibraryEventReceiver in Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c. Additional information is below"
while starting an Out-of-Box workflow.

The reason i found out after googling at lot was the AAM setting was messed up somehow.

I was all perfect though in tearms of accessing the site but i didn't noticed a bunch of Event
Log errors getting generated each day.

So fixing the AAM settings fixed the error for us.