The customer tried to implement RSU1807, but the system stopped at a very early IPL stage.
They used a separate SYS1.LPALIB.M that contained the CA Hooks for CA DISK.
When they excluded SYS1.LPALIB.M the system came up to a minimal state.
The SYS1.LPALIB.M is the first in their LPA concatenation. It contains the user modifications (CA DISK OPEN SVC ZAPs), while SYS1.LPALIB has the IBM standard modules.
The maintenance upgrade was going into SYS1.LPALIB.
Usually, if a maintenance upgrade changes any of the 4 CSECTs for the CA Disk OPEN SVC ZAPs, the SMP/E installation returns an error indicating to remove the user modification and apply the new OPEN SVC ZAP again.
However, RSU1807 changes only other CSECTs of modules IFG0194A, IGC0001I, IGGDADSM, IGC00020 than the 4 CSECTs needed for the OPEN SVC ZAPS.
So they had the new modules in SYS1.LPALIB, but still old modules with the OPEN SVC ZAPs in their SYS1.LPALIB.M.
Delete the contents of SYS1.LPALIB.M and generate it again with the new modules and the user modifications (CA DISK OPEN SVC ZAPs) applied.