We have the below custom button on opportunities:

{!REQUIRESCRIPT("/soap/ajax/19.0/connection.js")} 
{!REQUIRESCRIPT("/soap/ajax/19.0/apex.js")} 
var eQuoteEndpoint = sforce.apex.execute("UAUpdate","GenerateEQ", 
{user_id:"{!Opportunity.OwnerId}",opp_id:"{!Opportunity.Id}"});
//var eQuoteEndpoint =  //'URL REDACTED' + "token=//{!User.Quote_Token__c}" + "&opportunityId={!Opportunity.Id}";
console.log(String(eQuoteEndpoint)); 
window.open(String(eQuoteEndpoint));

Every so often it gives the below error:

A problem with the OnClick JavaScript for this button or link was encountered:

{faultcode:'soapenv:Client',
faultstring:'System.QueryException: LIst has no rows for assignment to SObject

Class.UAUpdate.GenerateEQ: line 9, column 1', }
 

As far as I can tell, it is referencing the below Apex class (only pasted until line 9 out of 184).

global class UAUpdate{
    public static boolean isApexTest = false;
    
    WebService static String GenerateEQ(String user_id, String opp_id){
      // Perform SOQLs to obtain relevant information.
      User user = [SELECT Id, Quote_Token__c FROM User WHERE Id = :user_id LIMIT 1];
      Opportunity opp = [SELECT Id, AccountId FROM Opportunity WHERE Id = :opp_id LIMIT 1];
      Account account = [SELECT Id, Name FROM Account WHERE Id = :opp.AccountId LIMIT 1];
      Contact contact = [SELECT Id, Email, FirstName, LastName, MailingCity, MailingPostalCode, MailingState, MailingStreet, Phone FROM Contact WHERE AccountId = :opp.AccountId LIMIT 1];

Is the class trying to reference a missing record/object? I can't tell what exactly it would be and how to fix it. Would appreciate any help.