Can't install on Shared Hosting

Hello all,
I have a problem when trying to install Litecart on my shared hosted package. 
Looks like there is a problem with connection to my DB although i checked several times and my credentials are correct.

Checking PHP version... 7.2.21 [OK]

Checking PHP display_errors... On [OK]

Checking $_SERVER["DOCUMENT_ROOT"]... /var/www/vhosts/test.com/sitexxx.eu [OK]

Connecting to database...
Warning: mysqli::__construct(): php_network_getaddresses: getaddrinfo failed: Name or service not known in /var/www/vhosts/test.com/sitexxx.eu/includes/library/lib_database.inc.php on line 12

Warning: mysqli::__construct(): (HY000/2002): php_network_getaddresses: getaddrinfo failed: Name or service not known in /var/www/vhosts/test.com/sitexxx.eu/includes/library/lib_database.inc.php on line 12

A few details that may help...DB name/DB username is long and contains _ (Comes from provider this way)

DB server: localhost:3306 
DB name: m46102nnic_crdeals
DB user: m4610_crdadmin

Any ideas/solution ? 
Is there any alternative/manual way to perform the installation ?

Best regards and keep up this ultrafast cart !
Don't put the port number in the server name.
Hello Tim,
I have tried this but nothing :( 
My localhost installation was just fine but ths i can not install.
If localhost can not be resolved to a destination then your webhost should tell you what the hostname is. If you leave the field empty LiteCart will attempt localhost.
Tim you didnt understand me, i was just saying my localhost (installation in my pc through wamp) is installing just fine.
I have some news....
I tried to replicate the problem and the same happened in my pc...
If the website files are located under a subdirectory you got the same error message...
i.e. files directory localhost/site.com/site2.com

UPDATE
the problem is not the directory itself but the username which contains an _ or many characters

UPDATE 2
the problem is not the directory itself but the username which contains an _
You posted an error that indicates the given hostname is not resolvable.

php_network_getaddresses: getaddrinfo failed: Name or service not known


Underscores in mysql usernames is very common and neither php or LiteCart should have a problem with these.
If you are putting it on shared hosting don't use localhost, put your mysql server url in there.  The _ in the names isn't an issue, all of mine are that way.