This issue has now been resolved
Please clear your cache and cookies if you're still experiencing this issue.
We are aware of an issue with access to your client's accounts. We are working on a fix with the highest priority.
What you can do in the meantime?
Copy this link http://c17.qbo.intuit.com/app/switchCompany?companyId=[COMPANYID]
And paste your client's Company ID in the link above to access your client.
To find the Company ID, please Select the client name in the client list, and then find the company ID in the URL.