Session timeout error - different flavor?

[holdech]

I upgraded a 1.0.13 forum to 2.0.15 a few days ago. It was a challenge but we got it done.
The upgraded forum worked perfectly for two days with multiple log in and log out etc.
Now I get the "Your session timed out while posting. Please go back and try again." error, even if I leave username and password blank.
I checked the theme index.template.php file (all themes) for the extra form fields and they were there for the default theme. I added them to the "core" file as well.
Cleared cookies and same result.
I also tried commenting out the checkSession function in LogInOut.php as recommended by another post.
That just made the system keep putting up the login screen.

What do I look at or try next?

Thank you

[holdech]

Forgot to add:
Php version: 5.6.3.1

You can try the forum at this url: hxxp:www.alohaowners.com/forum [nonactive]

You will see that any username/password (even blank) yields the timeout error.

Question - should I change the session handling from database to file (or vice versa)? I believe that's in the settings table in the DB?
Does the cookieName in settings.php matter?

[Sir Osis of Liver]

You will see that any username/password (even blank) yields the timeout error.

Not for me, brings up login window.  Have you changed anything?

[holdech]

I refreshed all files and database from backup copies.
Works now, but that's what it did previously - worked fine for a couple days and then stopped.
I'll see how it goes and either ask for help or close this topic.

The website that launches the forum was hacked with base64_decode. That site has been deleted and rebuilt from scratch but the forum was not.
I scanned for base64_decode and it only appears in the same places as in a fresh copy of 2.0.15 downloaded from this site.

It makes me wonder if there is any malicious code in the site. Not sure what else to look for?

[Sir Osis of Liver]

If you don't have a securuty scan in your host control panel, ask host to scan the account.

[Aleksi "Lex" Kilpinen]

I refreshed all files and database from backup copies.
Works now, but that's what it did previously - worked fine for a couple days and then stopped.
I'll see how it goes and either ask for help or close this topic.

How are things going, are we ready to close this or have there been issues still?