The location of log file is define during configuration.
Default location is projeqtor/files/logs (one file per day)

[root@pmsadmin logs]# cat projeqtor_20181119.log
2018-11-19 06:14:43.619 ** ERROR ** [] Exception-[42S02] SQLSTATE[42S02]: Base table or view not found: 1146 Table 'projeqtor727.parameter' doesn't exist
2018-11-19 06:14:43.619 ** ERROR ** [] For query : select * from parameter where (idUser is null and idProject is null)
2018-11-19 06:14:43.619 ** ERROR ** [] Strack trace :
2018-11-19 06:14:43.619 ** ERROR ** [] #0 Sql->query called at [/opt/pms/projeqtorV7.2.7/model/persistence/SqlElement.php:2371]
2018-11-19 06:14:43.619 ** ERROR ** [] #1 SqlElement->getSqlElementsFromCriteria called at [/opt/pms/projeqtorV7.2.7/model/Parameter.php:982]
2018-11-19 06:14:43.619 ** ERROR ** [] #2 Parameter->getGlobalParameter called at [/opt/pms/projeqtorV7.2.7/model/UserMain.php:143]
2018-11-19 06:14:43.619 ** ERROR ** [] #3 UserMain->__construct called at [/opt/pms/projeqtorV7.2.7/model/User.php:38]
2018-11-19 06:14:43.619 ** ERROR ** [] #4 User->__construct called at [/opt/pms/projeqtorV7.2.7/tool/projeqtor.php:3776]
2018-11-19 06:14:43.619 ** ERROR ** [] #5 getSessionUser called at [/opt/pms/projeqtorV7.2.7/tool/projeqtor.php:164]
2018-11-19 06:14:43.619 ** ERROR ** [] #6 require_once called at [/opt/pms/projeqtorV7.2.7/view/main.php:37]

It seems db is empty.
Check content of db.

That is as expected: This result is for a plain vanilla installation, which is supposed to have a empty database.

OK, so log should contain other lines...

No actually I pasted the entire content of that file.
Checked once again to confirm.
This is it!

Do you have any error logged in Apache error log file ?

Here's the Apache log file contents:

[ 2018-11-19 02:35:11.8891 10080/7fed42ca8700 age/Cor/SecurityUpdateChecker.h:466 ]: A security update is available for your version (5.1.2) of Passenger, we strongly recommend upgrading to version 5.3.5.

[ 2018-11-19 02:35:11.8892 10080/7fed42ca8700 age/Cor/SecurityUpdateChecker.h:471 ]: Additional information:
Several critical security issues have been fixed since v5.1.2, due to an issue in Passenger < 5.1.6 we can not display more info.

[ 2018-11-19 15:35:12.5808 10080/7fed42ca8700 age/Cor/SecurityUpdateChecker.h:466 ]: A security update is available for your version (5.1.2) of Passenger, we strongly recommend upgrading to version 5.3.5.

[ 2018-11-19 15:35:12.5808 10080/7fed42ca8700 age/Cor/SecurityUpdateChecker.h:471 ]: Additional information:
Several critical security issues have been fixed since v5.1.2, due to an issue in Passenger < 5.1.6 we can not display more info.

[Mon Nov 19 21:29:07.976750 2018] [autoindex:error] [pid 10117] [client 10.0.3.1:42582] AH01276: Cannot serve directory /var/www/html/: No matching DirectoryIndex (index.html,index.php) found, and server-generated directory index forbidden by Options directive

Just to clarify, we have a virtual machine that runs only ProjeQtOr, and nothing else.
And this is the log for that machine for that day.
So my engineer thinks that this "Passenger" must be a part of ProjeQtOr -- Can't be anything else...

Passenger is not part of ProjeQtOr.

Issue linked to projeqtor acces may only be the last one : no index page.
It means your vhost is not defined.
Possibly you try and access http://yourserver, you should try http://yourserver/projeqtor

Please post screenshot or what you call "work in progress"

I have asked our engineer to follow your tips and revert.

But then a question arises as to why this same system worked fine with all the earlier versions of ProjeQtOr?
Nothing was changed.
Has the new version changed in some respect?

No, nothing has changed on new versions that may explain issue faced.
But possibly it is how you migrate that can have negative impact.

My engineer has tried a fresh installation and the result is as follows.

1. After unzip, we get the configuration page.
After entering the new database details, we get the screen shown below:

The Webserver log is here

Note :- Even though it is a NEW installation, still it is referring as an "upgrade". Is there a clue in this?

The Database does not show any tables, as can be seen from the following:

mysql -uprojeqtor727 -pprojeqtor727 projeqtor727
Welcome to the MariaDB monitor. Commands end with ; or g.
Your MariaDB connection id is 795134
Server version: 5.5.52-MariaDB MariaDB Server

MariaDB [projeqtor727]> show tables;
Empty set (0.00 sec)

MariaDB [projeqtor727]>

**********
My engineer tried to install a "pure vanilla" version of the V7.3.1 RC, but he gets the same result.

Hi,

On install, ProjeQtOr does not know any credential except "admin".
You must connect as "admin" with password "admin" to finish the installation process.

oops. Wrong screenshot! My engineer had tried to log in as Admin.

Did you find anything in the log?

There si nothing in the Apache log file.
We need Apache Error Log and Projector Log files.

My engineer says that there is no other error from Apapche as the error is not from Web server side.

Nevertheless, if you still feel that there is another error log file, please guide me with likely location for this error log file.
Thanks in advance!

Apache error log file should be in
/var/log/apache2/error.log

Projector log in
/var/www/html/projeqtor/files/logs/projeqtor_{YYMMDD}.log

We have tried in a new server and result is same as given below:

No log files generated under log folder

error log is with 0 size in Webserver also.

One thing stands out is even in a fresh server's fresh installation, the on-screen message from the installer appears like a upgradation patch.
(This should happen only when we are running the installer on an existing installation.)

Message tells about ugrade because it is the upgrade of the DB.
It will migrate from V0.0.0 to V7.3.3 (you have this informatrion on top right of the screen).

1) Are you sure you connect with admin / admin ?
2) You must give write acces on log folder to PHP user (apache on centos). On your screenshot, owner is codendiadm, and right is 644 so apache cannot write on the log folder.

I had the same problem as described here. Really, I had to check the web server's error.log. It was referring to an error for starting the php session: PHP Warning: session_start(): Failed to read session data: files (path: /var/lib/php/session).

After I enabled the web server user to write to /var/lib/php/session, the "upgrade" was successful.