Good news.
I applied the method suggested by Mariosipad and tested the settings step by step.
Well... all woks as expected in the end !
I have
- the multiple database view
- the custom language file (because I don't like the French name of Collection for the 'Series'
- the htaccess password
- and of course the custom column (and even a different custom column for two different databases), that can be clicked and is displayed across all screens
In the end I can't tell what when wrong as the settings of the test setting seem to be the same as for the non-working setting.
So far this can be put on the account that I can have messed up something, somewhere, without knowing. Which would not be a surprise due to my poor (no) knowledge of php.
But there is more to this.
In order to do the testing I created a new COPS folder, named differently than the original non-working (I.E. '
COPSTEST' vs '
COPS').
So '
COPSTEST' works as it should and '
COPS' has the non-working custom column issue.
After this testing I cleaned up the Web folder and renamed the non-working '
COPS' to '
COPS_OLD' and the working one to '
COPS'.
Surprisingly the one now called '
COPS' (which has the new setting) get's the problem !
And the one now called '
COPS_OLD' (which was for far having a problem) works properly !!
Which means that the name '
COPS' seems to be the cause of the problem...
I tried other names around and in the end, any name works, except 'COPS'.
Of course I blamed the browser caches and tested the issue with different browsers, clearing the caches each time.
I even tested this on my wife's PC that has never been connected to the server.
I can reproduce the problem each time.
In the end, I can live with a folder named anything except 'COPS', but I would like to understand why the name of the 'COPS' folder can create this problem.
Finally, warm thanks to Mariosipad for having put me on the path of the troubleshooting process.