I accidentally found out what the problem is.
The link that this variable refers to:
var $tabValue = $(this).attr('href');
The hash at the end matters:
https://bupacouk.bwa.local.internal.bupa.co.uk/cash-plan-quote/quoteAction.do?getBenefitLevelDetails=getBenefitLevelDetails&productPolicyId=7850
This will cause AJAX to crash in IE versions in bothe.
Using the following code:
var $withoutHash = $tabValue.substr(0,$tabValue.indexOf('#'));
Getrs got rid of him and now it works :)