Upgrade 3.2.2 to 3.3.7 - Server internal error 500

[metalgalle]

Hi guys!
I'm using OTRS version 3.2.2 installed on a Windows 2008 R2 dedicated server.
Everything works fine, but I want to upgrade my installation.
I downloaded the lastest windows installer, and upgraded it.
Everything seems to get fine but, my index.pl page, after sending my user authentication data, respond with an internal server error 500.

I know that there are many topics and possible solutions, I searched for them and tried to apply, but the problem was never solved.

I can say that:
- MySQL is correctly up and running
- CheckDB.pl script returns "Connected" without errors
- Customer.pl page does not seems to suffer the problem
- Apache is correctly up and running
- Apache error log, says "Premature end of script headers" referring to index.pl page

Now I came back to lasto working configuration using VMWare snapshot and I'm already online, but I still wish to upgrade.

I tought that maybe the upgrade process left some errors because the gap between versions is too big, but I can't find where I can download old setup files in order to upgrade my system step-by-step...

Many thanx to everybody in advance!

Regards!