Error 500 After a Month of No Problems

[rammjet]

Updated March 30 from PrestaShop 1.6.0.9 to 1.6.1.4.  There have been no problems for the month since then.  Nothing has been changed in the store.

Suddenly, a few days ago, Error 500 - Internal Server Error started showing up in the back office.  Not everywhere, only a few places.

Clicking on Preferences or Advanced Parameters or Administration produces the Error 500.

Also, trying to duplicate a product causes the Error 500.

The above actions produce the following messages in the error log:
 

[Fri May 06 17:45:43 2016] [error] [client xxx.xxx.xxx.xxx] Premature end of script headers: index.php, referer: http://mydomain.com/myadmin/index.php?controller=AdminDashboard&token=some-token

[Fri May 06 17:46:36 2016] [error] [client xxx.xxx.xxx.xxx] Premature end of script headers: index.php, referer: http://mydomain.com/myadmin/index.php?controller=AdminProducts&token=some-token

The errors happen virtually immediately, so I do not believe it is a timeout issue.

I've turned on Dev mode in config/defines.inc.php and I cannot see any additional information anywhere.

I've scanned all files for permissions and all are 755 or 644 - there are no 666 files.

I've tried turning off Mod Rewrite in .htaccess

PHP version is 5.4.36

Scanned dozens of online postings with nothing quite like this.  Lots of "Error 500" and lots of "Premature end of script headers", but very few happening in the back office and none reported by clicking on the above items or by duplicating a product.

I'm hoping some suggestions will point me in the right direction.

Edited May 7, 2016 by rammjet (see edit history)

[tuk66]

The error message means that there is a text output before the HTTP header is sent. It needs some debugging as there is no visible issue.

[rammjet]

Thanks.  I've spent most of the day looking for where that might be happening.  No luck yet.

 

Plus, I'm having difficulty with idea that the shop was working fine after the update and then stopped working.  I cannot find anything that changed since the update.

[rammjet]

Thanks to tuk66, the problem has been solved.  HTMLPurifier was causing the problem.  Disabling it allowed proper function.