I have exactly same problem, but only on Packages second ISO file.

Getting SLE-15-SP1-Installer-DVD-x86_64-GM-DVD1.iso goes well (from cdn.microfocus.com)
Getting SLE-15-SP1-Packages-x86_64-GM-DVD1.iso causes this problem even on Linux/Firefox or Windows/Edge or Windows/IE11, it always fails at 54% for me (=~ 4.22 GiB)

Tried with wget, but from cdn.microfocus.com this is not easy, and not sure this is feasible ^^