The error message clearly states that the string "germany" is being used in the calculation.
Perhaps you could provide a link where I could see this in a browser to get a better feel of what is going on in the code.
Also try disabling the rule for the moment so we can troubleshoot what the session variable is really set to.
After disabling the rule, add the following after the body tag on the paymentdetails page add the following code to write to the screen the value of the session variable:
Session TaxRate = <%=Session("TaxRate")%>