We are still on Weblink 9 and have started getting errors where tiles fail to load, then an alert pops up with the following message: 

A lost connection condition has been detected. You may need to reload the page to continue.  Would you like to reload the page now?

Sometimes the error happens immediately on load, so it's not an issue of being idle. It only happens to some files, not all.

This happens on Chrome, Firefox, and Edge. I looked under Chrome dev tools and found these network errors:

You can see the broken tile images. At first I thought it was a permissions issue, but Weblink already has permission to view annotations. Also, you can see that some of the AnnotationInfo.aspx requests succeeded!

I also went on the server to look at Event Viewer, but Weblink isn't listed, only web client. I don't know where else to look.