it will affect the transactions, you need to make sure the version of the udf is matched before you input any transactions. 

Please check what is the version of your current UDF. Please do the below steps to check the version:

1. go to C:\Program Files (x86)\Firebird\Firebird_1_5\UDF and look for QnEFuncs.dll

2. select QnEFuncs.dll and right click and select properties

3. Go to the tab "Details" and check for the product version

Let's say the current udf version required for QNE 691 is 2008.5.0.656 , if your version is not the same, Please do the below steps again:

1. stop the firebird services in services.msc

2. Go to the working directory of QNE 691 e.g. C:\QNE and copy QnEFuncs.dll and paste it to C:\Program Files (x86)\Firebird\Firebird_1_5\UDF

3. Start the firebird services and try to login again.

Thank you :)