BSM Support Tip: GDE Open API not working correctly in BSM 9.26 (Known Issue)
Customers that use GDE Open API with web services probably will notice a problem retrieving data using wsdl files with this API.
Basically the problem is an HTTP 500 Internal Server Error, not matter the connection parameter that you use.
This have been already reported to R&D and there is a hotfix available, please contact support to check the issue and provide the hotfix if it is applicable.
You can check information about this here QCCR1I113700