Found the issue, in some of the servers recently we stopped the SSRS service and detached the ReportServer and ReportServerTemp DB from the SQL instances since we are not using SSRS. while running maintanence plan for transactional log back up in the instances, its trying to create transactional log backup for missing DB's, because of unavailability of the database its generating the above mentioned error.
Actually there is a maintanence plan for creating transaction log backup and it is pushed down from master server to all other target servers and option was set to database log backup task is "All user db's excluding system db's", this caused the problem.
Even if I remove the ReportServer DB's in the target servers, the maintanence plan in master server still trying to create a log backup for missing user db's in targer servers.
I just modified Maintanence Plan option to specific databases, still the error is NOT stopped generating. If I attached the ReportServer DB's again, its stopped generating the error.
Not sure, I think need to clear the entries in meta tables in target server related to maint plan... Any idea ?
Thanks for your guidance.