zDC was cancelled previously instead of shutdown. In a cancel situation, the zDC may not shutdown completely and that happened here. When the zDC was restarted, that invocation fails with the U100 seen.
Client should run the ZDCDELET program contained in the product AUTHLIB library.
//DELETE EXEC PGM=ZDCDELET,PARM=<zDC-name, such as MEPC>
//STEPLIB DD DISP=SHR,DSN=DT.R050100.S1.AUTHLIB
// DD DISP=SHR,DSN=DT.R050100.S2.AUTHLIB
The ZDCDELET job clears out control blocks that were left in storage by the zDC cancellation and allows the zDC to be initialized successfully..