Awesome tips @acromeenes! 

It's common that some users have multiple Intuit accounts. 

Your clients must sign in with the same email address associated with the Intuit Link. Once they're logged in, they either need to change their Intuit Account email address to match the one you have or you'll have to un-invite the email you have for them and re-invite them with the email address they're using for their Intuit account. 
To ensure they are logging into the correct account, share this article with your clients and let them know which email address you're using: Recover your Intuit Account if you can’t sign in

Other troubleshooting tips: 
-clear cache and cookies as mentioned above
-sign in using an incognito browser  
-If routed to the create account page, scroll down to the bottom and click on "Already have an account? Sign In" link.

Check out the Learning More About Intuit Link page for more articles & reply back if any of these tips worked for you.