I can't browse images to include in CKEditor when using CiviMail. When I click the browse button in the image window, the following pops up
[....]/civicrm/civicrm/packages/kcfinder/browse.php?cms=civicrm&type=images&CKEditor=crmUiId_1&CKEditorFuncNum=0&langCode=en
and get "500 - Internal Server Error" with suggestions that file permissions are not correct. Don't know what to check specifically, but all the resource URLS and directories are defaults and file permissions look generally OK.
I have WordPress 4.8.2 and CiviCRM 4.7.25. There are one or two similar posts about not being able to browse files, but none quite the same and answers not very specific about what what files to check permissions of. Nothing in the ConfigAndLog directory. Works fine on the demo WordPress site, but of course can't see permissions etc. I'm stuck!