I had an issue come up with my Magento API calls. They were suddenly returning a SOAP fault where they were previously working just fine.
SoapFault exception: [SOAP-ENV:Server] Procedure 'loginParam' not present
Also related: Magento SOAP 2 API Fatal error: Procedure 'login' not present
This above link what what clued me it - the object to string conversion error was being produced by my script in addition to the SOAP fault.
It was a very simple fix. Someone who had access turned off the "WS-I Compliance" in System > Config > Magento Core API and the "login" method expected a string instead of an object.
Now - to figure out which colleague did it... ;)