502 Bad Gateway errors

[ASTOKES]

Hi I've been running Plesk 12.0 on a Ubuntu 14.04LTS server for a few months now - I migrated a whole bunch of sites etc. from an older plesk installation. I don't know if this problem has just arisen or I have only just noticed it:
When I go to domain control panel, users and click a user (or a role) I get the following:
502 Bad Gateway
nginx

...the logs show:
[error] 20050#0: *1269 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: n.n.n.n, server: , request: "GET /smb/user/overview/id/28 HTTP/1.1", upstream: "fastcgi://unix:/var/run/sw-engine.sock:", host: "domain.com:8443", referrer: "https://domain.com:8443/smb/user/list"
..and for the roles pages, the same except for page change to ...../smb/role/list

All the rest of the control panel seems to work OK (I admit I haven't checked every single option)...

Any Ideas? (the sw-mariadb-client package seems to be up to date - this was the only fix that I found for anything remotely similar)
Unfortunately my server provider knows nothing about plesk and I am unable to make direct support requests as they own the license - sooo frustrating!

Many Thanks

 

[IgorG]

If KB article http://kb.odin.com/en/122078 didn't help I can only recommend to create a request to support team to do in-depth investigation to find the reason and to fix it. Please create a ticket to support at https://odin.com/hcap/support/request/ .
You may have free support, please check what kind of Plesk license you use for available support options at http://kb.odin.com/en/121580 .
If there’s no free support in your case, you can order Plesk per-incident support at http://odin.com/hcap/support/buy-support/. Support team will contact you as soon as purchase is processed, and they will do the best to resolve it.
If it is found that your problem was caused by product bug w/o available solution or workaround from Parallels, then your purchase will be re-funded.