Why does API OKS_ACTIVATE_CONTRACT_PUB.activate_contract() return 'E' but gives no error msg?

I'm using Oracle Service Contracts, 12.2.10.

Today I'm trying to call the API, OKS_ACTIVATE_CONTRACT_PUB.activate_contract(). Its return status is 'E', but the l_msg_data is 'Contracts QA process has completed successfully.'

How can it complete successfully, but exit with Error??? And in fact, the status of the contract did not in fact change. It was previously 'Entered', and remains so.

I'm calling the API as closely as I can to the example in the API document. Does anyone have an idea why the API claims there was an error, but then claims that all is well, without actually doing anything?