Error code 4600

Error code 4600

Hello, 
some time ago we have I have implemented integration with ZohoCRM. Yesterday our client reported that he cant integrate. I have checked it and I found an error:


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




I found it for example in this method: 
https://crm.zoho.com/crm/private/xml/Users/getUsers?authtoken=<valid-authtoken>&scope=crmapi&type=AllUsers

It is very strange because for the rest of clients it works correctly, only for one customer it fails... They generated a new authtoken and there is no change, the same error. 

Can you help me with that ?