Just ran into the same problem, thanks @mattab that was driving me crazy.

I have the same issue but cat piwik.old > piwik.php did not help :-/

@Geobert I have to admit a couple of times it didn't work for me either. However the files are being compared it seems unstable. Unfortunately I can't help you anymore than to say that I kept trying with it and eventually it would relent. A better approach is probably to delve into how the file comparison is happening in the first place.

@benjackson84 thanks for your answer, but keep trying what? Just refreshing the page? Or doing cat piwik.old > piwik.php and refresh over and over again?

using 2.14.3 BTW

@Geobert sorry "kept trying" was a bit vague. I ran piwik, then deleted the file then ran piwik again, then restored the file and ran piwik again, then added a new line to the file then ran piwik again, then deleted the new line and ran piwik again. It was a trial and error process (which I hate), I got the impression I was battling with a cache. Eventually it accepted the file.

Same error here, but I don't have an old piwik.php file. After using the latest beta release no problems anymore.

Fresh Piwik install on fresh Debian Jessie. Having a "Warning" during Piwik installation for this. Hope it won't bite later.

Please try with latest beta as hopefully the problem is fixed: http://piwik.org/faq/how-to-update/faq_159/

Could you please point me to verify this is a problem at all? How would it manifest?

In my case (Piwik 2.14.3, new installation) I can see in Apache's error log:

PHP Fatal error: Uncaught exception 'Exception' with message 'The configuration file {.../config/config.ini.php} has not been found or could not be read.' in .../core/Application/Kernel/EnvironmentValidator.php:64

And IMHO there was no instruction to create such a config file before going through the installation steps. Actually the installation process will create it. After it is created, there's no HTTP 500 error any more. So, just ignoring the warning seems to be ok.

I have installed via wget http://builds.piwik.org/piwik.zip && unzip piwik.zip

I've just run into this issue, running nginx + php-fpm on port 443, Centos 7 + Ajenti V.

Initially saw the 500 error on install, tried disabling the mod_ssl which is running on port 9000 for subversion but no change.

Having looked on my server (which I wget then unzip the file on) the piwik.php is showing as zero bytes.

Moved the file to piwik.old and then pasted into vi a copy of the source piwik.php, saved it and tested with a 200 result.

+1 Well done! thanks @mattab

@dcht00 it's not something you can always seen in an editor.^M
For example, vim autodetects the file format. Hiding the ^M at the end of each line.^M
It can be a result of using a dos formatted file on a *nix system.^M
^M
The file might look OK, but CR/LF is different.^M
Resulting in the non-linux artifacts left in the file.^M

works for me too. I created an empty piwik.php file, then copy and paste the content to this file. voila....the page loads with no errors.
I also ran chmod 775 piwik.php

Thanks,