HELP: Login issue

[SomeUser01]

I'm new to SMF and I've recently started using it.
I'm currently using SMF 2.0.14 and the login through the home page(on the top of the home page there is the login field) doesn't seem to work. Whenever I try to login through that I'm getting the error "Your session has expired", regardless of what I enter. I'm thinking it is because of a modification I've installed, but I've uninstalled most of them now and it still doesn't work. The ones I still have are
Country Flags 3.0
Custom BBCode Manager
Read the Rules Link above post button

I made a minor modification to the post php myself to allow passing of text via url when posting a new thread.
Now I'm wondering how to fix the login form without having to install the forum again.
Thanks for any advice you can give me!

[Illori]

https://wiki.simplemachines.org/smf/Login_error_2.0.14

[SomeUser01]

https://wiki.simplemachines.org/smf/Login_error_2.0.14

Thank you so much that really helped me out!