XML error: Empty document

I got the following error when accessing FreePBX:
FATAL ERROR
XML error: Empty document at line 1
How to fix this error?
system: Trixbox 2.8.0
Thank you.
__________________

I got the following error when accessing FreePBX:
FATAL ERROR
XML error: Empty document at line 1
How to fix this error?
system: Trixbox 2.8.0
Thank you.