Sometimes we get the following message in error.log and sometimes not. The reason is that the size of MaxRequestLen is not high enough:
mod_fcgid: HTTP request length 135714 (so far) exceeds MaxRequestLen (131072)
Add the following entry in the file: /etc/apache2/mods-available/fcgid.conf
raise the value from the default 131072 to 15728640 (15MB) or higher if needed.
MaxRequestLen 15728640