Hi!
Thanks for the update and good news that the problem has finally disappeared, albeit under somewhat mysterious circumstances...
I have just tested a fresh install of the plug-in on a J3.7.3 site with PHP7. You mentioned a possible SQL artefact. Well, the plug-in uses only one SQL table. If this table is removed, the plug-in will create a new one without a single hick-up.
It seems that we'll never know what caused the crashing of your site.
Regards

Sorry for the delay in getting back to you. Since submitting the problem post and getting back I took the following actions which may be of interest.

As I had had a full clean site installation on my local machine using xamp/php etc. I decided to try installing the JHacker Watch Standard on this local site and it worked perfectly without any problems. I then took the chance to install the program on the live online site – again the site crashed. I was able to get the site back up quickly by restoring the mysql database.

Since then I have cleared the live hosting of the site and uploaded the clean local version and the associated mysql database. This works perfectly well, so I have resolved the problem in a rather convoluted way, but I thought you would be want to be aware of the issue. I can only assume it was something to do with the database creation on the hosting platform?
At least the end result is… it works.

If the adjustments that you made were plug-in configuration adjustments, then that should not be a problem. But maybe the plug-in was trying to fill a database record while the table to put it in was not yet there. That should however result in a warning or error, not crashing the whole website.
Best practice is to first enable the plug-in and then configure it.

If the plug-in 'enable' setting in the extensions db table is set to 0, then it is disabled and cannot do anything.

Anyway, just let me know if you are ready to proceed.

Thank you for the reply and the offer to help - I am not going to be able to look at the issue for while as I will be away, but will be in touch on my return.
Having read your comments I am aware that I did make some adjustments to the plugin before I enabled it - could this have been the reason for the crash?
The site has been completely restored with a clean back up and a copy of the original database. If it is of any help I have looked at the old database and found that the 'plugin' and the element 'jhackerwatch' are already set to 0.
Thank you

Hello. Thank you for your message and for your interest in our plug-in.
I gather that you are using the JHacker Watch Standard plug-in (paid edition).
You did not mention the plug-in version, but you should make sure that you are using the latest version.
It is not clear to me how your website crashed: did you see any warnings/errors or just a blank page?

When you enable the plug-in, I mean just activate it using its default settings, it will not do anything special, except that it will create a database table in your website's SQL database. It will also send an email to the site administrator saying that a table has been created. Did you see that email?
You have to manually enable the user account and directory monitoring before the plug-in begins its 'real' work.

I have installed and tested the latest plug-in version (v2.1.0) on a standard live Joomla! 3.7.2 install (protostar template), using PHP 7.1.5 and also PHP 5.6.25. There was no problem with the install nor after the enabling of the plug-in. I quickly checked the user account monitor and it was working as expected.

You may be using a Joomla! set up that uses a special template and/or other 3rd party extensions (in particular system plug-ins) that could - in some mysterious way - interfere with the JHacker Watch plug-in. To-date we are not aware that such interferences exist, but we are ready to learn every day...

Since I want to help you, I propose that you give me a temporary super user access to the related website, so that I can have a look at its setup and investigate the issue. Of course I will not be able to enable the plug-in and see what it does if that locks out my further access.
If you are willing to provide temporary access, then please email the log-in details to webdesign(AT-sign)innato(DOT)nl. Do NOT post them here!

If you will be testing again, make sure you have direct access to the website's SQL database. If you are locked out, you can always disable the JHackerWatch plug-in by editing its record in the table named 'XXXXX_extensions'. In that table, look for the type 'plugin' and the element 'jhackerwatch' and then set the table field name 'enabled' to 0 (zero) and save the record. This diables the plug-in.

As a path forward, I recommend:
- Completely uninstall/remove the JHAcker Watch plug-in from your website.
- Check if you are using the latest version of the plug-in. If not: update!
- Install and enable the standard Joomla! 3.7.2 front-end template (Beez 3 or protostar).
- Reinstall the JHacker Watch plug-in. Do not enable it yet.
- Set up a direct SQL access to the website's database
- Enable the plug-in and if the website crashes, disable it via your direct SQL access
- Check if you see an additional SQL table named XXXX_plg_jhackerwatch_prg_data. It should be there because it must have been created directly after the enabling of the plug-in. If it is there, there should also be an email in your inbox announcing the creation of the table.

Regards an good luck!

I have recently had to successfully complete a rebuild on a website. I then installed the JHacker Watch program. On enabling the program the site crashed and I was completely unable to access the backend admin or the site itself. I have since reinstalled a backup and the site is back on line.
I would appreciate any thoughts on what the issue might be. I am running Joomla 3.7.2. (I have not reinstalled the plugin!
Thank you for any help you may be able to offer.