Which msdb table holds the specific list of databases to be backed up for the 'backup database task'?
None. Maint plans are SSIS packages. Follow the advice above about re-saving the plan. You should be able to toggle the list to 'all databases', save, then back to specific ones, select the ones you want and save again.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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