I'm seeing the same issue. Here's what I've tried
After restoring a DB backup, I still have the same.
Ran a complete restore of all the project files.
I've checked for errors in the WHMCS and Apache error logs. I've double-checked that all the data is there. But there are none.
After disabling all modules, the error persisted.
Solution
Then I tried another browser and it seems the issue is due to stale browser cache.
I'm seeing the same issue on a Development server.
It's been working for months and suddenly this morning I'm unable to view any client records. For me it started after I created a new Customer.
As with the above cases, all the client records in the database remain intact.
Just WHMCS code that seems unable to show the customer details after pulling the full list onto the screen.
I'll be restoring from a backup but consider this highly irregular.