Okay, I want to make sure I am understanding here.

Did you use SSMS to create the maintenance plan?

Did you use the maintenance plan schedule to schedule and create the agent jobs?

If so, then open the maintenance plan in SSMS. At the top, you will see a button to manage connections. Validate that the connection setup is correct for that server.

Next, from the agent job - validate the command line that actually runs the package. For maintenance plans, it looks like:

/SQL "Maintenance Plans\{your plan}" /SERVER {your server} /CHECKPOINTING OFF /SET "\Package\{sub-plan}.Disable";false /REPORTING E

The next step would be to validate that each task in the sub plan is enabled, and that the dependencies are set correctly. In other words, in a normal backup plan I would have an integrity check task - with a green line (on success) from that task to the next task (backup databases task), etc...

If all of that is correct - then recreating the plan is the only option. It is possible the plan was created pre-SP2 and never recreated later. Any maintenance plans that were created before SP2 came out have to be recreated because of the changes made at that time. It is also possible that someone with a pre-SP2 client created the plan - which is not compatible with post-SP2 maintenance plans.