Guessing the upgrade did iris start [instance] nostu to not run any user code on startup.
iris help start
                            **** iris usage ****
Syntax:
        iris start <instance name> [parameters]
Description:
        Run the instance's irisstart procedure to bring InterSystems IRIS up.
Optional parameters:
        quietly -> non-interactive, with minimal dialog
        nostu   -> don't run startup routine (^STU)
        help    -> list supported parameters
        EmergencyId=username,password -> start up in emergency mode
        Any other value is taken as a configuration file name with '.cpf' appended.
        If a configuration file is not specified, the file 'iris.cpf' is used.
Example:
        iris start mystuff quietly      <- uses file 'mystuff.cpf'
Notes:
        Some instances might not have support for a particular parameter,
        or a new parameter could have been added.  For a list of supported
        parameters for a specific instance, type:
                iris start <instance name> help

Looks like :Aliases were added IRIS 2021.1 - Aliases

I would not completely replace users with LDAP.

Database owner and admin should be local to the system.

Make sure your LDAP source is not a single server.

The recommendation is XFS from Supported File Systems.

The irisstat command with the -u option allows you to look at this without going into iris.

You can also look at locks in the portal - Monitoring Locks

I had to debug an application LDAP issue and I used to Apache Directory Studio.

It turned out that the user had been copied from one OU to another and not moved.

Can you login with iris session [instance] -U %sys?

From there you can use ^SECURITY to change the password.

One thing to check is use a text editor to make sure your keyboard is working by typing the password.

Not sure that you have seen that Intersystems deprecated SAM November 13th.

Log level is not always a good indicator of how big a deal what is being logged is.
For example:
2 [Generic.Event] Unable to allocate shared memory minimum - this error prevents startup so critical
1 [Generic.Event] MirrorServer: Lost ReadDmn (ACK) connection to async - loss of connection connection to a mirror doesn't rate as even a warning?

Any maximum you set less than free space on the filesystem will just hit the limit sooner.
The instance could use more RAM in the global buffer and maybe more filesystem space.
You should look at this and this.

 I would expect the WIJ to close to force it to the disk about every 80 seconds ie every write cycle.

my quick testing:

$while true; do
> ls -l IRIS.WIJ | cut -f 10-13 -d ' '
> sleep 40
> done
Oct 26 12:26 IRIS.WIJ
Oct 26 12:27 IRIS.WIJ
Oct 26 12:27 IRIS.WIJ
Oct 26 12:28 IRIS.WIJ
Oct 26 12:28 IRIS.WIJ
Oct 26 12:30 IRIS.WIJ
Oct 26 12:30 IRIS.WIJ
Oct 26 12:31 IRIS.WIJ
Oct 26 12:31 IRIS.WIJ
Oct 26 12:32 IRIS.WIJ
Oct 26 12:32 IRIS.WIJ
Oct 26 12:34 IRIS.WIJ
Oct 26 12:34 IRIS.WIJ
Oct 26 12:35 IRIS.WIJ
Oct 26 12:35 IRIS.WIJ
Oct 26 12:36 IRIS.WIJ

You should look at this - Write Image Journaling and Recovery

The WIJ can live in other places.

As a guess the the web page is not getting data updates.
What does ^MIRROR say is happening?

Have you checked if there are firewalls interfering?

So testing shows the old way was better in this case.

Failure
iris session ${INST} -U %sys << EOF

w "stopping the mirror 1 is sucess: " zw ##class(SYS.Mirror).StopMirror("MIRROR")

h

EOF

%SYS>

%SYS>
stopping the mirror 1 is sucess: "0 "_$lb($lb(2050,,,,,,,,,$lb(,"%SYS",$lb("e^Shutdown+7^MIRRORMGR^2","e^zStopMirror+4^SYS.Mirror.1^1","e^^^0"))))/* ERROR #2050: Mirror configuration not loaded */

%SYS>
Sucess
$ iris session ${INST} -U %sys << EOF
>
> zw ##class(SYS.Mirror).StopMirror("MIRROR")                                  <
> h
> EOF


%SYS>

%SYS>
stopping the mirror 1 is sucess: 1

%SYS>

Same either way :(
iris session ${INST} -U %sys '##class(SYS.Mirror).StopMirror("MIRROR")' ;echo $?
0

Thanks for this example.
I had used:

iris session ${INST} -U %sys << EOF

w "stopping the mirror 1 is sucess: " zw ##class(SYS.Mirror).StopMirror("PMIR") h
EOF