Cannot run code from this file in conjunction with non encoded files

Support & Bugs
1

When I try to visit my analytics I get this:

Fatal error: Cannot run code from this file in conjunction with non encoded files in /path/to/domain/lang/en.php on line 81

I have a backup of my database. Piwik worked fine yesterday, today I tried to log in and now it wont work.

2

This is because your sysadmin (or hosting provider) enabled both have APC and either Zend Guard Loader or Zend Optimizer. AFAIK these are not compatible with each other.