Custom plug-in generates error after Php update

We have a procedure to publish every week a news letter for our customers. Last week this process began to generate errors without us having changed the program. According to the isp this could be related to a php update..
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Documentation is available as a video walk-through on the process of publishing one article.

Environment:
- WordPress 3.1.1
- Akismet 2.5.4
- Custom Post Type UI 0.7
- MailChimp 1.2.6
- MobilePress 1.1.5
- ShareThis 5.4
- Sidebar Login 2.3.4
- WPML Multilingual CMS 2.0.4.1

More details about this project:

* Description of your needs: This project is to revise the procedure and repair the errors which do not allow us to publish our weekly post.

* Project for a site/blog: Changes to an existing one

* Type of project: Newsletter/Online brochure

* Description of the required templates: no templates needed, just to fix the one we have

* Additional information: We're looking for a long term relation and use this project as a trial for future reference and projects.