Thank you for providing a screenshot here in the Community. Allow me to chime in and help you fix the issue regarding the member list in QuickBooks Online.
You can perform additional troubleshooting to resolve the unusual behavior on your end. Ideally, the box on the left side shouldn't go white and show an empty list after clicking a member (see screenshot below).
Since the issue persists after clearing the cache, I recommend updating your browser. The problem issue happens when you're using an outdated version.
Also, I recommend switching to a different supported browser. This way, you can isolate and figure out if the problem is browser-related.
Please visit this link to know more about the supported web browsers: System requirements for QuickBooks Online, Accountant, Payroll, and QuickBooks Self-Employed.
Additionally, I've attached an article you can use to merge duplicate or activate an inactive member in QuickBooks: Add and manage customers in QuickBooks Online.
Please know that I'm just a post away if you need more help managing customers in the Online version. Take care.