So I fixed my problem. Here's how:

a:4:{
    i:0;s:23:"bandpress/bootstrap.php";
    i:3;s:56:"posts-from-single-category-widget/post_from_category.php";
    i:4;s:15:"something else ";
    i:5;s:68:"syntax-highlighter-with-add-button-in-editor/syntaxhighlighterpp.php";
}

I figured out every activated plugin is defined between the i and the ;, so by removing the following my error message dissapeared:

i:4;s:15:"something else ";

So I fixed my problem. Here's how:

a:4:{
    i:0;s:23:"bandpress/bootstrap.php";
    i:3;s:56:"posts-from-single-category-widget/post_from_category.php";
    i:4;s:15:"something else ";
    i:5;s:68:"syntax-highlighter-with-add-button-in-editor/syntaxhighlighterpp.php";
}

I figured out every activated plugin is defined between the i and the ;, so by removing the following my error message dissapeared:

i:4;s:15:"something else ";

[edit]

I should also have changed a:4 to a:3. I didn't and I ended up with a:0 { }. However, after reactivating my plugins I was back to the way it was.