Support

It's explained in https://www.akeeba.com/documentation/akeeba-backup-documentation/how-to-cloud-backup-basic-config.html 

Most failed uploads are caused by timeouts. PHP and your web server have time limits, i.e. the maximum time a PHP script may process data before the web server aborts it. Uploading the backup archives to cloud storage takes time, the exact amount of which depends on the size of the file and the network speed. If that time is over either time limit your backup will fail. The time limit and the bandwidth are beyond our control. The only thing you can control is the size. Many post-processing engines support chunked uploading (breaking up the uploads in smaller bits and having the remote server piece together the file) and you can change their chunk size. A chunk size of 5 or 10 MB works best in most cases. For those post-processing engines which don't have an option for chunked uploads you will have to change the Part size for split archives in the Archiver Engine options. Again, a value of 5 or 10 MB works best in most cases. This setting will split our backup archive into multiple files (same base filename, the extensions are .j01, .j02, ..., .jpa; or .j01, .j02, ..., .jps; or .z01, .z02, ..., .zip;), the maximum size of each one being the value of this setting. To restore these backups just place ALL of these files in the same directory and choose the main .jpa, .jps or .zip file: the other parts are discovered and extracted automatically.

WebDAV does not allow uploading large files in small pieces (chunks). It's all or nothing. Therefore you need to send a small Part Size for Split Archives as explained in the documentation page I quoted.