Hi I'm working on the integration between AvaTax and our billing system, using the Salesforce Api. 
We have changed to a newSalesforce Organization and are attempting to calculate tax on orders for Canadian customers. This has worked well on other Sandboxes we have tried.
Is there a  reliable way to know why the parametr is not working when sending the Http Request?
I am noticing that for this new Sandbox org we are reciving this message when calculating the tax (See log file segment below) Thanks:
 

DEBUG|{"type":"SalesOrder","lines":[\{"quantity":1.00,"itemCode":"43490","description":"Freight","number":"1","amount":0.00},\{"quantity":1.00,"itemCode":"79345","description":"Puggles Backpack","number":"2","amount":4.1677967793}],"customerCode":"4","companyCode":"awana1","code":"988103","date":"2018-04-25 10:29:52","commit":"false","addresses":{"ShipTo":{"region":"BC","postalCode":"V4P 1H5","line1":"2430 King George Blvd Ste 101","country":null,"city":"Surrey"},"ShipFrom":{"region":"IL","postalCode":"60173","line1":"1620 N Penny Ln","country":"US","city":"Schaumburg"}}}
10:29:51.0 (36237563)|CALLOUT_REQUEST|[1747]|System.HttpRequest[Endpoint=callout:AvalaraTax, Method=POST]
10:29:51.0 (40289120)|EXCEPTION_THROWN|[1747]|System.UnexpectedException: Invalid parameter value "fB0Ni1BVtLnbAD1xVS1udw==" for parameter "Decryption Exception".
10:29:51.0 (40477323)|FATAL_ERROR|System.UnexpectedException: Invalid parameter value "fB0Ni1BVtLnbAD1xVS1udw==" for parameter "Decryption Exception".

The Apex code that makes the call out to Avalara is this:

webservice static Result CalculateTaxOnOpportunity(string OrderId)
    {

        decimal TaxAmount = null;        
        // Try to
        try
        {
            // Convert the OpportunitySalesforceId argument provided to a Salesforce ID
            ID TempId = OrderId;
        // If converting fails, return failure and an error message.
        } catch(Exception ex) {  return new Result(false,'Please supply a valid Order (15 or 18 character) Salesforce Id. Error Message: '+ex.getMessage());  }
        
        try 
        {
            // Initialize the tax variable to zero.
            TaxAmount = 0;
            AvalaraTaxJSON Obj = new AvalaraTaxJSON(OrderId);
            if(Obj.companyCode != null || Obj.companyCode !='') {
                string jsonData = JSON.serialize(Obj);
                jsonData = jsonData.replace('AvalaraCurrentDataTime','date');
                jsonData = jsonData.replace('Avalara_Commit','commit');
                jsonData = jsonData.replaceAll('Avalara_SeqNumber','number');
                system.debug(jsonData);
                Http h = new Http();
                HttpRequest req = new HttpRequest();
                req.setEndpoint('callout:AvalaraTax');
                req.setBody(jsonData);
                req.setHeader('Content-Type','application/json');
                req.setMethod('POST'); 
                HttpResponse res = h.send(req);
                system.debug(res.getBody());
                JSONParser parser = JSON.createParser(res.getBody());
                while (parser.nextToken() != null) {
                    if ((parser.getCurrentToken() == JSONToken.FIELD_NAME) && 
                        (parser.getText() == 'totalTax')) {
                            parser.nextToken();
                            TaxAmount = parser.getDecimalValue();
                        }
                }
                return new Result(true,'Success, tax amount calculated',TaxAmount);
            }
            else 
                return new Result(false,'The Order must have a valid Shipping Country before calculating tax OR Country must be United States Or Canada');
        }
        // If an error occurred getting tax
        catch (Exception ex)
        {
            // Return failure and an error message.
            return new Result(false,'An error occurred when getting tax from the tax system. Error:'+ex.getMessage());
        }
        
        // Otherwise, return Success as true and the Tax Amount calculated by the tax web service.*/
        return new Result(true,'Success, tax amount calculated',TaxAmount);
    }

The AvalaraTaxJSON object looks like this:

public class AvalaraTaxJSON {
string code;
public string companyCode{get;set;}
string type;
string AvalaraCurrentDataTime;
string customerCode; 
string Avalara_Commit;
address addresses;
list<lineItem> lines;   
    public class address{
        public AddressCls ShipFrom;
        public AddressCls ShipTo;
    }
    public class AddressCls{
        string line1;
        string city;
        string region;
        string country;
        string postalCode;
        public AddressCls(string line, string city, string reg, string country, string zipCode){
            this.line1 = line;
            this.city = city;
            this.region = reg;
            this.country = country;
            this.postalCode = zipCode;
        }
    }
    public class lineItem{
        public string Avalara_SeqNumber;
        public decimal quantity;
        public decimal amount;
        public string description;
        public string itemCode;
        public  lineItem(string num,decimal qunt, decimal amt,string itemCode, string des){
            this.Avalara_SeqNumber = num;
            this.quantity = qunt;
            this.amount =amt;
            this.description = des;
            this.itemCode = itemcode;  
            
        }
    }
    public AvalaraTaxJSON(string orderId){
        
        string isoCode;
        //Constructing JSON with OrderLine Items
        lines = new list<LineItem>();
        list<Order> orderLst = [SELECT ID, Name,Shipping_Street__c, Shipping_City__c,Shipping_State__c, Shipping_Zip_Code__c, Shipping_Country__c, TempOrderNumber__c, LastModifiedById, Error_Log__c,Account.IntacctID__c, Company__c,
                                (SELECT Line_Type__c, PricebookEntry.Product2Id, PricebookEntry.Product2.ProductCode, PricebookEntry.Product2.Name, Quantity, TotalPrice, Quantity_Given_Away__c FROM Order.OrderItems) FROM Order WHERE ID = :string.escapeSingleQuotes(orderId)];
        if(orderLst!=null&& orderLst.size()>0){
            Order Ord= orderLst.get(0);
            if(Ord.Shipping_Country__c!=null && Ord.Shipping_Country__c.toLowerCase().contains('united states') || Ord.Shipping_Country__c.toLowerCase().contains('canada')){
                list<Country__c> countryObj = [SELECT ISO_Code__c FROM Country__c WHERE Name = :Ord.Shipping_Country__c];
                if(countryObj!=null && countryObj.size()>0){
                    isoCode = countryObj.get(0).ISO_Code__c;
                    
                }
                Awana_Settings__c custSetting =  Awana_Settings__c.getValues('AvalaraTaxCompany');
                if(custSetting!=null)
                    this.companyCode = custSetting.value__c;
                else
                    this.companyCode = 'awana1-sb2'; //default value
                this.code = Ord.TempOrderNumber__c;
                this.customerCode = Ord.Account.IntacctID__c;
                this.type = 'SalesOrder'; 
                this.AvalaraCurrentDataTime = String.valueOf(system.now());
                this.Avalara_Commit = 'false'; 
                addresses = new address();
                addresses.ShipFrom = new AddressCls('1620 N Penny Ln','Schaumburg','IL','US','60173'); //Default Address Present in Cast iron
                addresses.ShipTo = new AddressCls(Ord.Shipping_Street__c,Ord.Shipping_City__c,Ord.Shipping_State__c,isoCode,Ord.Shipping_Zip_Code__c);
                integer itr =1;
                for(OrderItem oli : Ord.OrderItems){
                    lines.add(new LineItem(string.valueOf(itr),oli.Quantity,Oli.TotalPrice,oli.PricebookEntry.Product2.ProductCode,oli.PricebookEntry.Product2.Name));                   
                    itr++;
                }
            }
        }
    }
    
}