I installed WHMCS 4.3.1 yesterday and had made no modifications - just added some clients as I've never used WHMCS before.
However, at the end of the day I got the email saying there was an update to 4.4.1 which I ran, it completed without errors, and immediately had the problem.
In addition I've now noticed that the "invoice" button on /whmcs/admin/clientsbillableitems.php has no text.
The lines read:
-------
With Selected:
<input type="submit" name="invoice" value="">
<input type="submit" name="delete" value="Delete">
-------
Wondering if I should just delete and reinstall...