Import failed! The uploaded file is not a valid message or mailbox file

Ubuntu 14.04
Dedicated server

I try to export/ import a mbox file from Horde (Cpanel @ Hostgator) to Roundcube (Sentora) but receive the following error:

Import failed! The uploaded file is not a valid message or mailbox file

I have the following settings in my php.ini file:
upload_max_size = 128M
post_max_size = 64M

If I do the same (same exported file) in Cpanel @Hostgator (Horde ->Roundcube it succeeds without any problem. 

Are there any settings I forgot on my dedi?

PS I can receive/ sent email from domain A to the Sentora Roundcube account.