message not being sent with large file attachments

Hi, I made a form where you can upload two images up to 4MB each, the server is configured to accept files up to 64MB and the max execution time is 300 seconds. However, if the files weigh more than 3.5MB each, the message is not sent and I get this error: “Failed to load resource: the server responded with a status of 404 ()”.
I tried to disable all plugins but without success, the hosting provider, after verifying that server settings are ok, suggested me to check plugin settings or contact the plugin’s authors. Do you have any suggestions for me?

P.S. WP version is 5.8.2 and CF7 version is 5.5.2

Thanks,
Nicola