I am moving from a stand-alone PC to a VM in Proxmox.
I researched in the forum and it appeared to be a simple case of doing an SQL dump from the old system and copying the entire contents of the /vaw/www/piwigo folder.
Build the new VM (Ubuntu in my case) and install the LAMP stack.
Apache tested OK with the default page.
I rebuilt the database with MySQLdump and all tables are now there with their contents.
I edited the apache2.conf file in sites-enabled and it now points to the correct location, /var/www/piwigo

I created users piwigo_user and www-data and just to try to get it working ended up making them both admins. I also, in desperation, changed permissions to the entire /var.www/piwigo tree to 777 but still can't get it to work. I'm sure it's a user or permission issue but I'm out of my depth and can't find what they should be.

I didn't get as far as getting any error logs, all I got was the following error, so now I'm stuck. Any ideas, should I start again?

Piwigo encountered a non-coverable error
[mysql error]
Access denied for user 'piwigo_user'@'localhost' (using password: YES)

#1     my_error /var/www/piwigo/include/common.inc.php(122)
#2     include_once /var/www/piwigo/index.php(11)