Support

The actual problem is that you are trying to upload 2GiB part files over WebDAV. Please remember that I have documented why this is a problem:

Before you begin, you should know the limitations. As most remote storage technologies, WebDAV does not allow appending to files, so the archive has to be transferred in a single step. PHP has a time limit restriction we can't overlook. The time required to upload a file to WebDAV equals the size of the file divided by the available bandwidth. We want to time to upload a file to be less than PHP's time limit restriction so as to avoid timing out. Since the available bandwidth is finite and constant, the only thing we can reduce in order to avoid timeouts is the file size. To this end, you have to produce split archives, by setting the part size for archive splitting in ZIP's or JPA's engine configuration pane. The suggested values are between 10Mb and 20Mb. Most servers have a bandwidth cap of 20Mbits, which equals to roughly 2Mb/sec (1 byte is 8 bits, plus there's some traffic overhead, lost packets, etc). With a time limit of 10 seconds, we can upload at most 2 Mb/sec * 10 sec = 20Mb without timing out. If you get timeouts during post-processing lower the part size.

If you read this documentation paragraph you will see that it explains what happens, why it happens, and what to do (lower the part size for archive splitting under the archiver engine options).

As to why it only happens when you include you images directory, I think that by now the answer to that question should be easy to deduce: because that's where the vast majority of your data lies in. When you take a backup excluding the images directory you have a much, much smaller backup which uploads over WebDAV without your server timing out. That's good. This is a known safe backup archive size for your configuration. Change your part size for archive splitting to the size of your backup minus the images directory and take a new full backup.