Hi there, 

Quiet a few years ago (prior to my time at this company) we had a custom object and a custom button created on it to work with Conga Composer to create an excel from a template. 

Last time I used it about a month ago it was working fine. I just went to use it a few days ago and its no longer working. When I am in Classic mode it seems like its working and says merge complete but doesnt actually download anything 

When I am in lightening it opens a new window with the below error: 


I then copied the URL from the button to see what error I would get and got the below: 


Currently the Button URL is: 
https://composer.congamerge.com/composer8/index.html 
?sessionId={!API.Session_ID} 
&serverUrl={!API.Partner_Server_URL_80} 
&id={!Variation__c.Id} 
&ReportId=[VarItem]00O90000005hUbP, 
[ReferAndSign]00O90000007biaJ, 
[CoS]00O90000008nQWR?pv0={!Variation__c.Contract_of_SaleId__c} 
&TemplateId=a0M9000000CZz1y 
&DS7=13 
&OFN={!Variation__c.Contract_of_Sale__c}+-+{!Variation__c.Name} 
&OCNR=1

We have other custom buttons with Conga Composa that are working fine, its just this one and it was working previously. 

I have been using Salesforce for a while but never really on the developer side - any ideas on whats suddenly wrong with it would be great! 

Thanks, 
Anne