I am guessing that you are running something "around" or "on top of" WebSphere Application Server.
What I see is what you described and what you showed me (SystemOut.log) ... what that resolves to is that there is a port on WebSphere Application Server that is now configured for SSL traffic and is open and waiting for an SSL connection but instead receives a non-SSL packet (plain text) and so it complains about that.
There are only a few lines in that SystemOut.log that are not about the plaintext, and they should be reviewed
[6/3/21 18:16:04:060 CDT] 00000063 LTPAServerObj E SECJ0369E: Authentication failed when using LTPA. The exception is com.ibm.websphere.wim.exception.PasswordCheckFailedException: CWWIM4537E No principal is found from the 'admin' principal name..[6/3/21 18:16:02:140 CDT] 00000062 WrappingFileO W switchFiles TRAS0031W: Unable to rename file C:\IBM\WebSphere\AppServer9\profiles\eprofile3\logs\server1\SystemOut.log to C:\IBM\WebSphere\AppServer9\profiles\eprofile3\logs\server1\SystemOut_21.06.03_18.16.02.log during log file rollover. Attempting to copy file contents.
It is not real clear to me but I think the user registry (Where the userids and passwords etc. are stored) is the file based user registry, and there was a search done for the user 'admin' and it was not found...
And there is something weird going on in the Windows OS not being able to rename a file.
Maybe you are using something like RAD or Eclipse to start (And connect) to the server and you have not told RAD/Eclipse that the server is now secure?
Maybe the way WebSphere Application Server is being started is not with Windows Administrator rights, and that is why the file can not be renamed?
WebSphere System Test here. I believe this is a known issue; the workaround is to create a custom property in the Security section of the admin console:
Security > Global Security > Custom Propertiesname: com.ibm.websphere.security.useAllSSLClientAuthKeytypesvalue: true
And restart the deployment manager and affected node agents.