Do 8197 free/busy errors appear in your application log every 25 minutes? The actual text states:
Event Type: Error Event Source: MSExchangeFBPublish Event Category: General Event ID: 8...
Updated Jul 01, 2019
Version 2.0
Blog Post
Impressive. Now I finally understand why I was getting this. The servers were in a site with DCs from the Root and we had a trust issue.