Error 500 on Joomla 1.5 website

Hello,

I have used Mobile Joomla since 2010.
This week I upgrade to the latest version. It was working on Firefox, at home, and I set up a nice new mobile version.

BUT some people told me that my website was showing an Error 500.
So I tested it on IE and Chrome, got the Error 500.
I refresh the Firefox cache and cookies and finally got the same error.

In PHPMyAdmin, I set the MobileBot plugin to 0 instead of 1, I got the website back.

How can I use Mobile Joomla on my Joomla 1.5 website ???

What do you have in error_log?

I don't find the Error log. I am not sure I can access it.
There is nothing in the logs files of my website.