Hi glgeorgiou,
I’ve tested your snippet, adapting it to the following to make it valid:
<p>[tabs title="" disabled="false" collapsible="true" active="0"] [tab title="Tot (4-6 ετ?ν)"]</p>
<p><img class="alignleft size-full wp-image-116" title="Τοτ1" alt="TECHNODrive" src="https://techno.edu.gr/wp-content/uploads/2013/07/tot-1.jpg" width="61" height="56" /></p>
<h4>TECHNODrive</h4>
<p>Σε αυτ? την εν?τητα οι μαθητ?? θα μ?θουν για τα Μ?σα Μεταφορ?? και του? Καν?νε? Κυκλοφοριακ?? Αγωγ??:</p>[/tab]
[tab title="Another tab"]asdasdasd[/tab][/tabs]
It worked fine in both WordPress 3.5.2 with Squelch TAAS 0.3.1 (the latest stable version) using the TwentyTwelve theme with no other plugins active, and in WP 3.6.1 with both the TwentyTwelve theme and the TwentyThirteen theme.
First of all please try following the troubleshooting guide here: https://squelchdesign.com/wordpress-plugin-squelch-tabs-accordions-shortcodes/#troubleshooting
If none of the steps in the troubleshooting guide work then please email me a full copy of the code you are using in the page so I can test it, along with details of: The version of WP you are using, the theme you are using and any other plugins you are using alongside Squelch TAAS, plus a link to a page on your site that exhibits the issue. I’ll take a look into the issue and issue an update to the plugin if necessary.