Db2 maintenance appears to have caused a memory overlay issue with the Sysview/Db2 tool. IBM identified apar ST12088 as the solution but they may mean SO12088. Is ST12088 part of R20?
Release : 20.0
Component : CA SYSVIEW Performance Management Option for DB2 for z/OS
ST12088 is an R19 APAR. Its published version is R19 SO12088. SO12133 is its R20 version.