This error can be presented if the Microsoft .NET Framework has been updated to 4.6.2 or above. 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.

If this alternative does not work to resolve the error, you will need to downgrade the .NET Framework to 4.6.1. After downgrading, you may need to revert the application pool settings in IIS.