I have a few custom BBcodes defined long ago. After upgrading to V3.3.5, all of them worked as expected except for the [youtube] BBcode. I tried running the reparser with the same result.

To further debug, I installed a fresh version of V3.3.4 and V3.3.5, then defined the custom BBcode as follows:

Example: 

[youtube]http://www.youtube.com/watch?v={IDENTIFIER}[/youtube]

HTML replacement:

<style>.embed-container { position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden; max-width: 960px; height: auto; margin-top: 8px; margin-bottom: 8px; } .embed-container iframe, .embed-container object, .embed-container embed { position: absolute; top: 0; left: 0; width: 100%; height: 100%; }</style><div class='embed-container'><iframe src='https://www.youtube.com/embed/{IDENTIFIER}?rel=0&showinfo=0' frameborder='0' allowfullscreen></iframe></div>

Help line:

[youtube]http://www.youtube.com/watch?v=nnn[/youtube]

The "Display on posting page" is selected. It worked as expected in V3.3.4. In V3.3.5, the BBcode wasn't recognized. In the database, it is stored as follows:

<r>[youtube]<URL url="http://www.youtube.com/watch?v=DWmGArQBtFI">http://www.youtube.com/watch?v=DWmGArQBtFI</URL>[/youtube]</r> 

For V3.3.4, a similar post is stored as follows:

<r><YOUTUBE content0="54L1SwwGL8E"><s>[youtube]</s>http://www.youtube.com/watch?v=54L1SwwGL8E<e>[/youtube]</e></YOUTUBE></r>

For V3.3.5, the URL of the YouTube link is shown as an automatically parsed URL. I tried disabling that feature in the post ("Do not automatically parse URLs"). The result was the same.