This error can be presented if the Microsoft .NET Framework has been updated to 4.6.2 the following steps can be taken to resolve the error:

1. Access the Blackbaud NetCommunity web server. 
2. Navigate to Program Files > NetCommunity > admin > Events 
3. Edit the file Event2PriceSummary.ascx
4. Find line 25: 
ID="lblAmt" Text='<%# FormatCurrency(Decimal.Parse(Eval("GiftAmount"), Globalization.CultureInfo.InvariantCulture))%>' />
5. Replace this line with the following:
ID="lblAmt" Text='<%# FormatCurrency(Decimal.Parse(Eval("GiftAmount"), System.Globalization.CultureInfo.InvariantCulture))%>' />
6. Save the file.