It is impossible to download operating system images from cockpit-composer. After a few megabytes, the download slows down and eventually halts.
This has been reported and fixed upstream:
This got fixed upstream now: https://github.com/cockpit-project/cockpit/commit/a3fe60b761931
I will pull this into the 7.6 package as a patch.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.