I am getting the above mentioned error while executing subscription on Alert trigger.
I have not seen this error before. Can you provide a little more information about the Subscription and Alert involved? Perhaps if possible even provide the Subscription Service Code for review.
Logs would also be helpful here. Specifically the ApplicationLog and ErrorLog with error stack tracing enabled. You can enable error stack tracing by navigating to LoggingSubsystem > Configuration > Enable Stack Tracing.
What does your code look like?
If you have found a solution to your problem, we would be happy for you to post it here for the benefit of others on the community.