If you see the below listed symptoms then the root cause for CMDB and AtriumCore related plugins is a missing jar file in the "deploy" folder of AR Server:

File name: "cmdb-9.1.00-SNAPSHOT.jar"
Missing from: "BMC_AR_SYSTEM_HOME\deploy folder"

Symptoms to verify the condition:

AtriumPluginSvr log has this error: "RPC failed with: ERROR (8203): The server's RPC version is not supported."

arrecond.log shows "Connection Refused AR ERROR 90" (no jobs will run)

No cmdb_eng_debug_log.log gets created in BMC_AR_SYSTEM_HOME/db folder (absence of this log is almost 100% confirmation this condition exists)

Unable to reach ARServer with CMDBDriver. All external CMDB API's will show AR ERROR 90 instead of connecting. This can include Web Services and Atrium Integrator.

Caused by DB ARSCHEMA migration install without CMDB binaries and running AtriumCore install with environment settings to ignore AR load app:

BMC_LOADAPP_SKIP=TRUE
BMC_AR_LOADAPP_SKIP=TRUE

These environment variables prevent the jar file from loading but help us achieve to lay down configuration files and install binaries for DB migration without binaries.
 

This knowledge article may contain information that does not apply to version 21.05 or later which runs in a container environment. Please refer to Article Number 000385088 for more information about troubleshooting BMC products in containers.

To restore it please extract it from
AtriumCore/Disk1/files/cmdb/componentCMDBLinux.zip
or
AtriumCore/Disk1/files/cmdb/componentCMDBWindows.zip

and place it the deploy folder. It does not need to be executable, but read permissions should match those of the "CURRENT_USER" that starts AR Server process.
When deployed correctly you should see CMDB REST API and other CMDB related messages in the standard output of the load process (visible in Linux/Solaris only)