I don't know the answer but here are some shots in the dark:
- If you have not made any changes to the code, then download it again and compare if any files have been accidentally modified.
- Did you edit qa-base.php? There is an eval() call at 720 and a count() call at 750. If you have edited the file, then probably there is some missing quote or parenthesis or semicolon.
- Are you using any third party plugins? Couple of them I got are old and don't work well with 1.8.6. I had to make changes to them to make it work. Although, I did not see this particular error. You can try disabling the plugins one by one and see when it starts working.
- Use error_log(print_r($<variable>,true)); to print the 'variable' in question to make sure everything is in order. You'll have to enable LogLevel to 'debug' in apache2.conf and restart the service.