We have an integration solution that is used by over 200 customers. We send a standard call to test that the connection to the client Zoho instance is valid:  https://crm.zoho.com/crm/private/xml/Info/getModules?authtoken=<auth_token>&scope=crmapi where <auth_token> is replaced by the token provided by the customer. This works for the majority of customers, but for a couple we are now getting the following error returned:

4600 Unable to process your request. Please verify whether you have entered proper method name,parameter and parameter values.

This must be a configuration issue as it still works for the majority of our customers. Other forum discussions have talked about a syntax error but there is none in this case. 

What could be the cause of this error and how can we let our customers know how to fix it?

Thanks very much indeed.

-------*************UPDATE**************-------

We have set up another client today who has just generated a ZSC and we have the same error with them. We still have the large majority of our clients working. 

Anyone have any ideas that we could follow up on?

Thanks very much indeed.

-------*************FURTHER UPDATE**************-------

I decided to regenerate our very own ZSC key which was working without a problem and now we get the same error. 

When I then made the call with our old ZSC key we also got this same error. It did not tell us that our old key was now invalid.

Help would be very much appreciated.

Thanks