avast reports virus on my website but I can´t detect any infection. Please check

A customer using avast was so kind to report virus infection:
Please check www.mastering-academy.de and www.mastering-academy.com

My virus program and and online virus scan can´t find any problems.
But I would like to get rid of that problem.
Can you give me advice which file may be infected or is causing this problem so that I can figure out to remove it manually.

Thank you for your support.

Friedemann Tischmeyer

what does the message from avast say?

https://sitecheck.sucuri.net/results/www.mastering-academy.de/
https://sitecheck.sucuri.net/results/www.mastering-academy.com/

HTML code scan

https://www.virustotal.com/en/file/28a0a37fa9f541b102bda88a2cf49bba1d19bc675f5c8445409d42b442143d90/analysis/1455276500/

https://www.virustotal.com/en/file/c2d2a39bd0173e99c6183e20b6887fe969c8f31a661c7aa3c55e4fc0b496cc6f/analysis/1455276569/

I see two vulnerable libraries flagged here:
-http://www.mastering-academy.com
Detected libraries:
jquery-migrate - 1.2.1 : -http://www.mastering-academy.com/wp-includes/js/jquery/jquery-migrate.min.js?ver=1.2.1
Info: Severity: medium
http://bugs.jquery.com/ticket/11290
http://research.insecurelabs.org/jquery/test/
jquery - 1.11.3 : (active1) -http://www.mastering-academy.com/wp-includes/js/jquery/jquery.js?ver=1.11.3
jquery.prettyPhoto - 3.1.2 : (active1) -http://www.mastering-academy.com/wp-content/themes/infocus/lib/scripts/prettyphoto/js/jquery.prettyPhoto.js?ver=2.3
Info: Severity: high
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2013-6837&cid=3
Info: Severity: high
https://github.com/scaron/prettyphoto/issues/149
https://blog.anantshri.info/forgotten_disclosure_dom_xss_prettyphoto
(active) - the library was also found to be active by running code
2 vulnerable libraries detected

Isue and warning here: https://mxtoolbox.com/domain/www.mastering-academy.com/

This link? → http://toolbar.netcraft.com/site_report?url=http://online-sale24.com is flagged:
Javascript included from a blacklisted domain. Details: http://sucuri.net/malware/entry/MW:BLK:2
Javascript: online-sale24.com but it does not resolve now, no response: https://urlquery.net/report.php?id=1455281096382
So I see no active malware now.

CMS issues to mitigate:
WordPress Version
4.4.1
Version does not appear to be latest 4.4.2 - update now.

WordPress Plugins
The following plugins were detected by reading the HTML source of the WordPress sites front page.

jquery-featured-content-gallery latest release (1.2)
http://www.cibydesign.co.uk/resources-and-downloads/
all-in-one-seo-pack 2.2.7.5 latest release (2.2.7.6.2) Update required
http://semperfiwebdesign.com
facebook-likes-you 1.5.4 latest release (1.5.4)
http://wolnaelekcja.pl/wp-facebook-likes-you

Warning User Enumeration is possible
The first two user ID’s were tested to determine if user enumeration is possible.

ID User Login
1 None wp_admin :o Should come disabled.

Another issue: 50% of the trackers on this site could be protecting you from NSA snooping. Tell mastering-academy.com to fix it.
Insecure Identifiers
Unique IDs about your web browsing habits have been insecurely sent to third parties.

d5fb79cb40414a3091dxxxxxxxxae2ac1a1445965753 local.adguard.com __cfduid

At least 8 third parties know you are on this webpage.

-Google
-Google
-Facebook
-Google
-www.mastering-academy.com
-online-sale24.com (this was at the culprit of your malware infection, Avast flagged as HTML:Script-inf, aka Sucuri’sreports at: http://sucuri.net/malware/entry/MW:BLK:2 )
-local.adguard.com
-www.mustbebuilt.co.uk -www.mustbebuilt.co.uk

polonus (volunteer website security analyst and website error-hunter)

F-Secure lab confirms infections

==========================================================
The file you sent (academy.com) was found to be malicious.

We will be detecting the sample you submitted as Trojan.HTML.Agent.MG

The file you sent (academy.de) was found to be malicious.

We will be detecting the sample you submitted as Trojan.HTML.Agent.MF

It says: Infection Type: HTML:Script-inf
The complete message is German. It says that the URL has a malicious code

Well, the infection has been confirmed, you’ve to clean it.

if you need help Sucuri will do it for a fee >> https://sucuri.net