System Monitor web page: initial login issues

What you can do is to review the database SentinelDB and the table User to view what user is created by the install.

In my case the default user is the local account Administrator, after login I will be able to add more users in the Configuration section in the portal.

What I can imagine is that reinstall does not re-create the SentinelDB and might be locked to the user linked on 'first' installation.
I have not verified this though.

Thanks for the tip !

Reinstalling does not re-create the SentinelDB, and I cant login with the one user created by the first install (domain account, tried current and former passwords).

Is there a safe-ish way to clear/delete the SentinelDB to start fresh again?

If you have access to the database you could actually edit the user table, modifying the username to your current and available user. (no password is stored in the database).

It has to be in the format of domain\username or if it is a local account computername\username.

Dropping your SentinelDB will require you to taking it offline and close all existing connections and for some reason does a repair not re-create the database.
I have to check if there are any other options to get this recreated, I would guess a full uninstall / install would do the trick but it feels like a quite brute force action to resolve such a simple issue.
Start with modifying the user table and let see what we can find.

I would believe Techsupport has better insights in the proper actions for this scenario.

Edit: As you mentioned, Un-installing System Monitor Manager component from Apps and Features and then installing it again using SP 2023 media, selecting Modify and then adding System Monitor Manager again did not create the database in my test either.

Edited the user to another domain account and it worked, that was simple, thanks a lot !

I wonder if the issue was to do with the first account password containing some UTF8 characters (é è à)

I'm happy that it worked out!

For future reference, I did manage to have the installer re-create the sentinel database, but I had to remove the SentinelDB from my SQL Server, also deleting the SentinelDB.mdf and SentinelDB_log.ldf from my SQL Directory. If the files exist, the installer will not re-create them and make the database available.

Taking the DB offline and dropping it will not remove the files.
Fastest way is to Detach it and then delete the files.
(default location: C\Program Files\Microsoft SQL Server\MSSQL16.MSSQLSERVER\MSSQL\DATA)
Then I could remove the Sentinel Monitor Manager using Apps & Features

After this has completed I could add it again by initiating the System Platform 2023 R2 Install, selecting Modify

Finalizing the install will recreate the database and make it available for the sentinel portal.

I hope it might help someone else in a similar situation 

I'm happy that it worked out!

For future reference, I did manage to have the installer re-create the sentinel database, but I had to remove the SentinelDB from my SQL Server, also deleting the SentinelDB.mdf and SentinelDB_log.ldf from my SQL Directory. If the files exist, the installer will not re-create them and make the database available.

Taking the DB offline and dropping it will not remove the files.
Fastest way is to Detach it and then delete the files.
(default location: C\Program Files\Microsoft SQL Server\MSSQL16.MSSQLSERVER\MSSQL\DATA)
Then I could remove the Sentinel Monitor Manager using Apps & Features

After this has completed I could add it again by initiating the System Platform 2023 R2 Install, selecting Modify

Finalizing the install will recreate the database and make it available for the sentinel portal.

I hope it might help someone else in a similar situation