Thanks Poul, this is a bug.
You can fix it on your own board by editing line 106 of admin/doapproveusers.php:
admin_log("USER_APPROVED", '$uid');
The single quotes around $uid need to be removed:
admin_log("USER_APPROVED", $uid);
The same issue effects line 120:
admin_log("USER_REG_DENIED", '$uname');
turns into:
admin_log("USER_REG_DENIED", $uname);
I've filed this bug internally.