I recently had a frustrating problem where I couldn’t upload files into MailWizz host.  This also causes problems with command line CSV imports. I discovered I had to open up /etc/my.cnf and edit the following line to

max_allowed_packet = 110M

then in /etc/php.ini I changed the following two lines
post_max_size = 200M
upload_max_filesize = 500M

then execute

service mariadb restart

Under settings, import/export on the backend I set CLI Import Enabled to Yes and Url Import Enabled to Yes.