Why are we getting this error after wiredTiger upgrade?

[Snorre]

I'm trying to upgrade from splunk 8.1 to 9.0 on a single server Windows installation. Upgrading the kvstore from mmapv1 to wiredtiger caused me some headache, but it eventually seems to work. The problem now is that I'm stuck on wiredTiger 4.0, and can't find a way to get it upgraded to 4.2. splunkd.log contains this error message:

08-03-2022 07:58:43.566 +0200 ERROR KVStoreBulletinBoardManager [7688 MainThread] - Failed to upgrade KV Store to the latest version. KV Store is running an old version, service(40). Resolve upgrade errors and try to upgrade KV Store to the latest version again.

But I can't find anything else useful in the logs.

splunk show kvstore-status --verbose

Any hints as to how to find out whats wrong? Or how to force the upgrade? I tried to delete the mongod-4.0.exe but that caused the kvstore to fail at startup.

[Mlac]

Same issue  here.
Upgraded from 8.1.3 to 9.0.1

08-30-2022 16:25:49.014 +0200 ERROR KVStoreBulletinBoardManager [166467 MainThread] - Failed to migrate KV Store storage engine to WiredTiger. This prevented running the latest version of KV Store. Migrate KV Store storage engine to WiredTiger and then upgrade KV Store. The Mmapv1 engine is deprecated.

Managed to set  engine to wiredTiger and status reports a OK, but  message in webGui state 

Failed to upgrade KV Store to the latest version. KV Store is running an old version, service(40). Resolve upgrade errors and try to upgrade KV Store to the latest version again.

when I gave command:

splunk show kvstore-status --verbose

I got  listed following  Info:
serverVersion : 4.0.24

after executing migration withouth type of storage engine:
splunk migrate migrate-kvstore

I got upgraded version
serverVersion : 4.2.17

Now Web GUI lost  error and all works  fine !

I  used  solution from the  following  post:
https://community.splunk.com/t5/Installation/How-to-upgrade-Mongo-in-Splunk-9-0-0/m-p/603185#M11666