Unable to generate thumbnails for illustrator(.ai) files

If you are able to view contents in illustrator, but can't preview the same file in Eagle, then the reason could be version or compatibility of the .ai file in question.

How to address the issue?

The reason for this issue is related to the compatibility selected when saving the .ai file. You can open the ai files by illustrator and save the file again with the "Create PDF Compatible File" option checked. The compatibility issue can be solved in this way.