2- Try deactivating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.
3- If that does not resolve the issue, try switching to the default theme for your version of WordPress to rule-out a theme-specific issue (theme functions can interfere like plugins).
4- Check the server error log to know the error detail of this issue.
Thanks
Mohammad
Clarifications: this issue appeared only on my Macintosh (localhost – Apache 2.4.25 \ PHP 5.6.30), I didn’t face with such a problem on host.
Solution (it worked for me):
1. Logout from admin profile
2. Give read/write permission to all files and folders in project folder