I've had this problem before (about 3 times now) Version 7.10 Mendix. I had to delete my local copy and download a new one. I've had the problem again later same as you and decided I wasn't going to delete and download again.
What I tried then was to unpack the widget.mpk it was complaining about.
In my case it said: could not find part of path: blablabla Drawline.js I believe it was.
I then went to the widgets folder and unpacked the Drawline mpk I found there into the folder.
Hope this works for you too (if you decide to try it)
I filed a support ticket to Mendix and they think this problem occurs when you make use of folder redirection or server back-ups.
"I checked with our R&D Team and it was confirmed that network shares have too much latency for Desktop Modeler to work correctly. As a work around it is recommended to map a drive letter to the mounted file system so that the paths looks like this: `Z:\Documents\...`."
"I checked with our R&D Team and it was confirmed that Desktop Modeler is intended to use the local hard drive for file storage and only when it is installed on Parallels on Mac the "network share" is supported (in truth the Parallels network share is still the same computer)."
Personally, this didn't help me very much as even the local drive of my laptop is back-up-ed by our servers, but maybe this could help you.
I got the error message as well. Even for widgets that I do not use (anymore) in the project.
In my case the problem lay in the browser cache. It only occurred when I opened the local application in Firefox. When I opened the app in Chrome it worked ok.
The resolution was to delete all cookies and website data for localhost in Firefox and now the error is gone.