Hi
KamalG,
You might consider to ask your server administrator, to have a look into your log and configuration - files. He can surely explain, why "such things" happen. He could as well explain to you, if the issue/failure is caused by Plesk itself, or by a package on your server, shipped by your vendor - with your informations we can only guess what happens on your server, which is quite inefficient for us.
If you would like to investigate the issue on your own and would like some help from the Odin - Forum community, it really would help to post some log - entries, because "502 Bad Gateway" - errors are pretty common and can be caused for several reasons. Maybe it might help to google the words "502 Bad Gateway", so you might understand, that it is nearly impossible to investigate issues, if you don't provide more informations.
You really should consider to bookmark the page:
... because this will help you to find log - files and configuration files for your Plesk installation.
As well, if this is a "fresh" installation, be aware that some vendors offer operating system installation templates, which are not really up-to-date, or the templates miss essential packages, which are needed for your server ( and Plesk ).
And last... Plesk invented a pretty nice tool called "bootstrapper", which finds and corrects misconfigurations on your server and each bootstrapper procedure includes a pretty informative log - file, which will point to issues/problems/mistakes/failures, which have mostly nothing to with Plesk itself.
/usr/local/psa/bootstrapper/pp12.0.18-bootstrapper/bootstrapper.sh repair