[Security!] Very important. Users posting with others usernames [?!!??!!]

[Mupetz]

Hello,

I don't know how can this happen.
Users are posting something and they realize they are on another member account !!
This is quite weird.
Example :
On forum home last post appears made by X
When you view topic it shows made by Y, but the post belongs to X ! ( X made it ! )
How can this be possible ?

The thing is ... i installed the script 13 jan. Yesterday 14 jan i added the mods and the people begin to register. 13 jan no people to post. So how can i know if its the script or the mods ?
2 Members had this problem since yesterday. Total 27 members ( almost all posting ).

Yesterday i installed several mods
Including VBSEO

I already thought about deleting all mods, but i spent all day installing more than 30 modifications.
Uninstalling all of them and installing again will take even more ..

Do you have an idea of what file, template, sql query, anything may cause this bug ? Maybe i can identify the modification causing this problem.

Please reply, this is very urgent.
Thanks