During AR Server upgrade (to 18.x, 19.x, 20.x), only FileDeployer process is started not ARSERVER and upgrade will fail.  Installer log may show any one of the below messages:

1. See error 'Could not start AR Server in timely manner'

Below is from arsystem_install_log.txt:
..............
SEVERE,com.bmc.smbu.install.common.rule.engine.ar.state.change.UnixArStateChangeStrategy,
  LOG EVENT {Description=[Failed to start service ],Detail=[Could not start AR Server in timely manner]}
SEVERE,com.bmc.install.task.InstallationStateHelper,
  LOG EVENT {Description=[AR System Server  failed],Detail=[featureARServer]}
CONFIG,com.bmc.install.task.InstallationPropertiesHelper,
  LOG EVENT {Description=[SET PROPERTY FAILED_INSTALL_FEATURES],Detail=[featureARServer]}
SEVERE,com.bmc.install.product.arsuitekit.platforms.arsystemservers.arserver.ARServerStartRuleEngineInstallerExecutorTask,
  LOG EVENT {Description=[[wait=2400000,exitCodeInterpreter=STANDARD]],Detail=[java.lang.Exception: Could not start AR Server in timely manner]}
..............


2 See 'Return code 90' repeated for next 2 hours (but no 'Could not start AR Server in timely manner')

Below is from arsystem_install_log.txt:
**********
  PROGRESS EVENT {Description=[arserver.ruleengineexecutor.installer.description],Progress=[40],Detail=[Adding Plug-in Server to ARMonitor.......]}
  PROGRESS EVENT {Description=[arserver.ruleengineexecutor.installer.description],Progress=[50],Detail=[Adding DSO Server to ARMonitor.......]}
  PROGRESS EVENT {Description=[arserver.ruleengineexecutor.installer.description],Progress=[60],Detail=[Starting the AR System Server services......]}
  LOG EVENT {Description=[Starting main services BMC Remedy Action Request System Server server123]}
  LOG EVENT {Description=[Current BMC Remedy Action Request System Server server123 state],Detail=[Stopped]}
  LOG EVENT {Description=[Executing RIKJ command = PING -B true -s 0 -C true -t 6020 -u Demo -x server123 -l E:\Apps\AR System -L true -n ARServer-RIK_PostInstall ]}
  LOG EVENT {Description=[Execution of RIKJ command PING resulted in Return code 90]}
**********

Check the following:
1- No other rogue processes is running (armonitor.exe, ARMONI~1.EXE, arsvcdsp.exe, etc....)
2- If the process 'arfiledeployer' is the only one running.
3- No other Java processes are running (ARServerJ, Plugin server, Atrium, etc....)
4. Check arfiledeployer.log see 'Patch Mode' is set to 'true':
     [main] INFO  com.bmc.arsys.filedeployer.ARFileDeployerDaemon  - Patch Mode  is true
5- Review the  ARMonitor.properties file which gets created under ARSystem/bin (this file is present from AR 9.1 onwards only).
     Check the parameter:
     com.bmc.arsys.armonitor.serverpatchmode
     to see if it is set to true or false.
     Example:
     com.bmc.arsys.armonitor.serverpatchmode=true

This parameter gets set during any upgrade to 9.x version, and the installer is supposed to take care of resetting it back to 'false' but failed to do so for some reason.

To perform a successful upgrade, perform below steps:
1- Stop AR Server Services before starting the upgrade.
2- Check and ensure that all process are stopped. 
3- Run the upgrade in PAUSE mode.
    Refer to the article:
    Remedy - Server - How to install ARServer using Pause Mode
4- In the first pause, nothing needs to be changed.
5- In the second pause, check the ARMonitor.properties created under ARSystem/bin. 
    Make sure this parameter present like below, if not set it:
    com.bmc.arsys.armonitor.serverpatchmode=false
6- Continue the pause and observe on screen where it will show 'Starting AR Server'.
    At this step check the file again and confirm its not getting set to 'true'. If it does, set this to 'false'.
7- Manually re-start AR Server Services (stop Service and confirm no java process are running and then start the arserver services)

Post these steps the upgrade should continue without any issues.