A lot of people who have different Google accounts for YouTube and Adsense have trouble connecting the two. In the case of a second channel within an already approved account this can be very frustrating. Plus YouTube support is extremely slow in responding and does not provide with the solution.
It took me 5 days to figure this out, after trying all possible fixes out there, from trying to login by cross-copying the url into another browser or a private tab etc. I tried enabling and updating all possible browser components, flash, java, disabling plugins, using 5 different browsers and ultimately 3 other computers. Deleted cookies, removed Google accounts from the browser. All these fixes don’t work because you should not login from YouTube into another Adsense account. The trick is in linking your YouTube account to Adsense BEFORE attempting to log into Youtube. Here is the solution in detail:
First of all don’t try to log in to your Adsense account from the monetization panel of your YouTube Channel. Whatever is going to happen in your monetization panel will happen while you are logged in your YouTube account.
There are two solutions here:
1) If your YouTube account is not an administrator to your Adsense account, then you have to set it an administrator. Log into your Adsense account and go to Settings > Access and Authorization. Enter the email address of your YouTube account and press “Invite”. Now log to your gmail with your YouTube account (or to the email you have chosen for that). You must have received an email by Google inviting you to be an administrator to the Adsense account. Click on the link to accept the invitation. Then go to Adsense with your Adsense account and tick the box to enable the YouTube account as an administrator. After that, login to YouTube with your YouTube account, go to the Monetization panel and enable monetization for your channel with your YouTube account (if asked to login, login with your YouTube account, not with the Adsense one). And this should work.
2) If your YouTube account is already an administrator to your Adsense account, and you are trying to add Adsense to a second channel, then you have to do a little trick. The trick is to login to Adsense with your Adsense account, remove your YouTube account as an administrator and re-add it. 😀 You have to go through the process described in the previous paragraph again, to re-add it as an administrator. This should work!