Lightning app preview error

I've been working though the Lightning trailhead modules. I know it used to work but the TWS app preview window now always fails with "Uncaught TypeError: Invalid value used as weak map key throws at https://....dev-ed.lightning.com/auraFW/javascript/-...../aura_proddebug.js:16252:26"
Not sure if it was installing R3 or something else I did to caues this but the preview always gives me this error now.
Is this known? Is there a logfile or something that I can look at to debug this a bit more?
Also: ctrl-c doens't work on the error window but (imo) it should.

Hi Dave,

Thank you for your bug report.
The error in the Lightning previewer that you are faced with means that there is some mistake in your javascript code. Looks like, this applies to your javascript which causes the functionality from the Lightning Application Bundle that contains the 'aura_proddebug.js' file.
You can open your Salesforce Org in Google Chrome and follow the next steps to get an additional information:
1) please open the Org's page that contains the Lightning Bundles that cause the error in TWS Lightning previewer
2) please click F12 and open 'Source' section;
3) there you can find the 'Pause on Caught Exceptions', please enable this and refresh the page.
The exceptions will appear in the Console, and they can be related to the reason of failed displaying Lightning Application in TWS previewer.
You can find TWS log file using the next way: please open the Main Menu -> Help -> Report a Bug. There you will see the links to the Activity log file and to TWS Application Logs. You can send us a bug report using this form, so we will be able to analyze errors from your logs if they are.

Please let us know if these steps help you to get an additional information about the issue. Our developers will investigate this also.
Thank you,
Kate