Blog » 500 Internal Server Error Courtesy of the Zend Optimizer

500 Internal Server Error Courtesy of the Zend Optimizer

Last night I went to log in to the CMS administrator pages for this website in order to add to this website. To my horror, I was greeted with a 500 Internal Server Error. This was quite puzzling since it had worked fine up until that day. For the next several hours, emails flew back and forth between technical support and me, as we collectively searched for a solution. I suspected that they had changed something on the server that resulted in this problem. However, they stated that they had not made any changes for several weeks. The error looked serious, since PHP performed a 48 MiB core dump every time that page was accessed.

Eventually, I found the culprit: the Zend optimizer. Adding the following line to the .htaccess file solved the problem:

php_flag zend_optimizer.optimization_level 8

Yes, the Zend optimizer has some obscure bug in optimization level 9 that just happens to be triggered by the Silverstripe log on page. What most likely happened, was that Zend was indeed updated several weeks previously, but the server had continued to use the cached optimized code until yesterday. 

Unfortunately, lowering the Zend optimization level did not solve everything.  Being relieved at seeing the log in screen, I was next confronted by an empty administration page. This turned out to be corrupt data in the silverstripe cache. For anyone else experiencing this problem, the cached admin pages can be flushed by visiting:

Alternatively, delete the files in the silverstripe cache manually.

This website is now almost fully functional again. I say almost because file uploads no longer work. Selecting a file will no longer add it to the upload queue. Fortunately files can also be uploaded via ftp and are listed in the CMS after flushing the admin assets page, e.g.:

This minor problem is something that I can live with for now. I would like to thank the staff for responding within minutes to my call for help, even though I ended up finding the solution myself; if only all companies responded so quickly.

EDIT: This was due to inadvertently deleting the base URL configuration  setting that is required as is documented in my blog entry on installing Silverstripe on

Blog » 500 Internal Server Error Courtesy of the Zend Optimizer

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Blog » 500 Internal Server Error Courtesy of the Zend Optimizer