SQL Server 2012 :: TRANSACTIONS After Moving To A New Server

I have a piece of code that has happily run daily for at least 4 years. I have moved it to a machine that is meant to have an identical set-up, and it fails. I need to work out why it fails, but I am stuck. The code works as follows:

CREATE storedProc1 (Parameter1, etc)
DECLARE @TransactionName nvarchar(127), @TransactionActive bit
-- Plus Other Variables...
BEGIN
BEGIN TRY

SET @TransactionName= 'TransactionName1'
SET @TransactionActive = 'True'
BEGIN TRANSACTION @TransactionName WITH MARK

[Code] .....

When I EXECute StoredProc1, it fails with the Error Message "Cannot roll back . No transaction or savepoint of that name was found." I think that message comes from a ROLLBACK TRANSACTION statement in the CATCH of StoredProc2. I have tried commenting out that ROLLBACK statement, which changes the error message to: "Transaction count after EXECUTE indicates a mismatching number of BEGIN and COMMIT statements. Previous count = 1, current count = 2."