Failed opening required 'sites/all/modules/og/og.field.inc I was trying to add the BoWoB Chat module to my site. It doesn't work and the error log file in its directory contain multiple lines like the following ones: 


  Failed opening required 'sites/all/modules/og/og.field.inc' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/public_html/mysite.com/sites/all/modules/og/og.module on line 16


Lines 15 and 16 are the following ones.  

// Load all Field module hooks for Organic groups.
require(drupal_get_path('module', 'og') . '/og.field.inc'); 


The file og.field.inc is in the same directory as sites/all/modules/og/og.module and its permissions are 644, so it is readable.

This one has me stumped. Do you have any suggestion?
Your file at least needs to have 755.

With 644 it is readable, but is not executable so the file cannot be managed as a php file (executable).