well i have a similar error with scheduled Tasks > 30 days (vCO 5.1 / 5.5 / 5.5.1).
e.g. i have a workflow that exports every day a statsictic about the virtual machines.
That workflow works for ~ 1 months and than it failes because the credentials were not correct.
If I edit that Task - enter same Password again, the workflow will run for the next month and fail then again.
Today i detected a setting in vCenter that could cause this Problem:
Log in to vSphere Web-Client -> Manage -> Single Sign On - Configuration.
On Policy - there should be a register card "Token-Policy" The last Option of that Register Card should be something like "Maximum lifetime of a token of type "holder-of-key"" - that value is set to ~ 30 days.
I changed that value to 10 years and will see what happen...
Maybe that adress your problem, too.