The integration appears to be working fine for us now. When clicking the "log into GTW" from within Admin and a blank window pops up/disappears, that could be an indication that the connection is fine. If not - let's say the password changed - they you would have an opportunity to provide updated login credentials.
Ours is working again also. Had to follow the steps Dan mentioned. Thanks for your help Dan.
I, too, have been having this same issue since 3/14. It caused a huge problem with a recent webinar and we were unaware of the issue because Marketo never provided a notification that the API connection was broken. We've since tried to re-establish the connection and continually fails. I'm almost glad to see others are experiencing this -- we were going crazy trying to troubleshoot.
I am going to assume you've fixed it by now - but for anyone else having the same issue (I just had it today, Oct. 24) take these steps to correct it.
When I would click LOGIN TO GTW - it would look like it was processing, then return to the same error (API/403) under details. Combining things above - I simply:
1. Opened an incognito window
2. Logged into Marketo
3. Logged into GTW from Launchpoint
Closed the window and the error was gone - I returned to the non-incognito window, hit refresh and it was resolved there as well.
I suspect this was due to someone changing the password on the account.