Unfortunately I that exception message doesn't tell me exactly what went wrong. The log file (C:\ProgramData\Solarwinds\InformationService\v2.0\Orion.InformationService.log) probably would.
Looking at your code, I can make a guess about the issue: you have added several extra fields to the AlertInfo class and removed the ObjectId field. That class is used to pass the arguments to Orion.AlertStatus.Acknowledge. When that verb does not find the arguments in the expected format (three fields: DefinitionId, ObjectType, ObjectId), it fails.
It is working fine.