Issue Nginx error 504 Gateway Time-out

[WhiteTiger]

I have installed OSTickets and am configuring the mail settings.
If I save them with SMTP disable, I have no errors.
If instead I save them with SMTP enabled and its settings, then I get the error "504 Gateway Time-out"
In the SMTP settings I just put the mail domain (domain.tld), port 465, username and password.
In the log I find this messages

2021-02-02 18:40:18    Access    MyIP    200    GET /support/scp/emails.php HTTP/2.0    https://mydomain.tld/support/scp/emails.php    Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.104 Safari/537.36    3.16 K    nginx SSL/TLS access
2021-02-02 18:40:23    Access    MyIP   200    GET /support/scp/emails.php?id=1&_pjax=%23pjax-container HTTP/2.0    https://mydomain.tld/support/scp/emails.php    Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.104 Safari/537.36    2.69 K    nginx SSL/TLS access
2021-02-02 18:41:43    Error    MyIP    504    POST /support/scp/emails.php?id=1 HTTP/2.0    https://mydomain.tld/support/scp/emails.php?id=1    Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.104 Safari/537.36    562    nginx SSL/TLS access
2021-02-02 18:41:43    Error    MyIP        38686#0: *3853 upstream timed out (110: Connection timed out) while reading response header from upstream                nginx error
38686#0: *3853 upstream timed out (110: Connection timed out) while reading response header from upstream

==== UPDATE
I replaced the SMTP settings.
Instead of using the domain SMTP on Plesk I used that of a GMail account.
The configuration did not generate the TimeOut and then the mail was delivered without problems.
So, the problem is only when I refer to the SMTP configuration of the account on Plesk

 

[Bitpalast]

Maybe the SMTP access credentials that you have supplied are incorrect so that a script is waiting too long for a response from the SMTP server?

 

[WhiteTiger]

No, it is certainly not a problem of incorrect credentials because the Webmail works.
I tried to install osTicket on another server without Nginx and only Apache2, but always to read the folders managed by Plesk.
Now I no longer have the 504 error, but it still doesn't accept the credentials.