There are two places where the OTF library may throw out the 413 error:
https://github.com/OpenThingsIO/OpenThings-Framework-Firmware-Library/blob/master/OpenThingsFramework.cpp#L102
https://github.com/OpenThingsIO/OpenThings-Framework-Firmware-Library/blob/master/OpenThingsFramework.cpp#L120
If you try the request in the browser, do you still get the 413 error? I feel it may have to do with you using a socket for the GET request.