We recently upgraded to version 4.6.11 in our test site - https://stage.npnparents.org.  We need to upgrade several apps/plugins.  When I go to https://stage.npnparents.org/admin/?app=core&module=marketplace&controller=onboard, click on an app, and  and click on "Sign in to Marketplace", I get an error after inputting my email address and password:

There was an error authenticating with the Marketplace. Please try again later or contact technical support for assistance.

Please advise.  Thank you.

 

Your staging site is behind a password prompt, you would need to temporarily disable that whilst you sign in. Alternatively, you may be able to exclude <url>/index.php?app=core&module=system&controller=marketplace from your password prompt.