Hi folks
We upgraded from AGE 11.2 to AGE 11.5
Portal, ArcGIS Server, relational ArcGIS Datastore & tile cache ArcGIS Datastore on separate VMs (VMWare)
Portal content folder, arcgis server directories & config-store on a fileshare.
Upgrade of Portal went fine
Upgrade of ArcGIS Server was not succesful

I found another post (Solved: Upgrade step 2 of 2 : Failed. Refresh auto-deploye... - Page 2 - Esri Community) that suggested to try another browser. However as ArcGIS Server was completely broken already, we needed to restore the snapshot of the VM. As the Portal upgade was succesful, I decided to only let restore the ArcGIS Server VM. After the restore we had an 11.5 Portal federated with a 11.2 ArcGIS Server. ArcGIS Server directories and the config-store on the fileshare were reverted to the 11.2 files.
Next, I did a second attempt to upgrade the 11.2 ArcGIS Server. But I saw an error in the ArcGIS Sever logs:
<Msg time="2025-09-15T16:36:34,941" type="SEVERE" code="6618" source="Admin" process="11988" thread="1" methodName="" machine="xyzxyzxyz" user="" elapsed="0.0" requestID="">Failed to return security configuration. Cannot invoke "com.esri.arcgis.discovery.admin.store.ConfigStore.securityConfigLastModified()" because "this.d" is null</Msg>
The timestamp of the error matches this step in the upgrade process:

When arrived at the Continue Server Upgade step, it failed again but other behavior now. After clicking Continue Server Upgrade, the process immediately terminated (1 second) and an authentication-prompt appeared. Seems that the federation / communication between Portal and ArcGIS Server was not OK anymore and after digging in the logs I found the error above. Most services are missing.
I try to understand what went wrong the 2nd time: error seems to point to the security-config.json, but that file was just present on the fileshare

No idea what the error (Cannot invoke "com.esri.arcgis.discovery.admin.store.ConfigStore.securityConfigLastModified()" because "this.d" is null</Msg>) tries to say
Any idea what might be the issue? 11.5 Portal federation with 11.2 ArcGIS Server validated well (only complains about the version not matching beween Portal and ArcGIS Server but this makes sense during an upgrade).
What do you usually do when an ArcGIS Enterprise upgrade fails in a later stage and the different components are on several VMs? As Portal upgrade went fine I decided to only restore the ArcGIS Server VM and to retry only the upgrade of ArcGIS Server (Portal upgrade took already 1.5 hours). Restore all VMs and start all over again with the Portal upgrade?
Best regards
Phil
@JonathanQuinn @Scott_Tansley