502 Bad Gateway is shown while accessing Plesk.
[error] 2194#0: *11 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 220.127.116.11, server: , request: "GET /left.php3 HTTP/1.1", upstream: "fastcgi://unix:/var/run/sw-engine.sock:", host: "panel.domain.tld:8443", referrer: "https://panel.domain.tld:8443/
mariadb-server 5.5.36 is used for Plesk database.
From strace log, you may see the following error:
"symbol int2str, version libmysqlclient_18 not defined in file libmysqlclient.so.18 with link time reference", 107}
MariaDB bug: MDEV-5773 .
Downgrade Mysql server to 5.5.35 version as it is said in MDEV-5773 .
Also libmariadbclient18 package should have the same version:
# dpkg -l |grep libmariadbclient18
ii libmariadbclient18 5.5.35+maria-1~wheezy amd64 MariaDB database client library
Make sure that mentioned symbols in error exist in
# strings /usr/lib/libmysqlclient.so.18 |grep libmysqlclient_18