I am using Sugar Community Edition, Version 6.2.0RC3 (Build 6350).
I had a bunch of modules in a custom package, renamed some of the modules and then tried to modify the Sugar Dashlet Layouts. I got this error message any time I tried to save the layout: Unable to load Module Dashlet Definition.
There is no easy fix. Prevention is better than the cure. Sorry. :(
From the post unable to load Module Dashlet Definition on the Sugar forums, you can supposedly fix this by renaming a bunch of files and modifying all references to those files. Perhaps because I have a package, I couldn't even find a match for the directory structures referenced in the post.
So, I am going to re-build the entire package. Luckily, I am still prototyping, so I have the scope for doing this.
What I have learned: don't rename a module, just change the label.