Two things and I'll mention them here as part of general analysing GAS logs and sending information to support.
1. When analysing issues, run the GAS logs with CATEGORIES_FILTER (
http://4js.com/online_documentation/fjs-gas-manual-html/#gas-topics/c_gas_asref_LOG_CATEGORIES_FILTER.html) set to ALL, this will give you most level of detail in your logs. It can also provide detail on the values in your configuration.
2. END_URL (
http://4js.com/online_documentation/fjs-gas-manual-html/#gas-topics/c_gas_asref_APPLICATION_END_URL.html) is defined as part of your GAS or application configuration in a .xcf. An error message about the value of END_URL suggests to look at the value defined in the .xcf and make sure the value is what you think it is. If you don't have configuration details in logs, then should also look at and send .xcf files.
Reuben