Well, I was working on upgrading from 2.3.x to 2.5.x. Installed OpenJRE 21. Made a backup copy of my entire jenkins folder. Problems with 2.5.x, so I stopped the Jenkins process.
Renamed
c:\ProgramData\Jenkins to Jenkins_New
c:\ProgramData\Jenkins_Copy to Jenkins
Started Jenkins
Wants me to login. Wrong passwor. 
Set useSecurity false and restarted Jenkins.
still wants login and none of my credentials will work.
Note: JRE8 is still there for old jenkins, UNLESS 21 is overriding something?
Bit more info. Our company split off from another company in 2020. This doesn’t have a lot to do with anything since it has been running for 5 years, but I just looked at the jenkins\users directory. Ouch. Time to do some MAJOR CLEANUP.
Are you using configuration as code plugin? If yes this will potentially overwrite your <useSecurity>false</useSecurity> change in the config.xml
Are you using Java 21 after restoring JENKINS_HOME from backup? An old 2.3xx version might not work with Java 21
I think I’m going to do the smart thing and continue on getting it running in 2.5.x with Java 21. And ask questions about why the NEW version is grumpy. 