Your Basis team hits a wall calculating the software needed for an upgrade in the Maintenance Planner — where do you look first?

At the LMDB.

The LMDB and Maintenance Planner should show the same information; a wall usually means the LMDB data is wrong or not regularly synced. Check that the LMDB's technical system is complete and synced first. (SolMan setup schedules a daily job to auto-update the Maintenance Planner.)