If you are experiencing this problem, please have your Salesforce administrator set the 8x8 Call Center option Enable Salesforce SSO to false until the issue is resolved. This will allow users to log into the 8x8 integration manually.
When logging in to the 8x8 Salesforce Integration with SSO enabled, you receive a blank pop-up window or the following errors: "Invalid Credentials" and "Fail logging into Salesforce. This is expected behavior for user emulation."
This appears to be a problem with specific Salesforce servers. Salesforce has been informed, and is reported to be working on this issue.